The "working-ground" (kammatthana) of the five hindrances

"There is the case where a monk remains focused on mental qualities in & of themselves with reference to the five hindrances. And how does a monk remain focused on mental qualities in & of themselves with reference to the five hindrances? There is the case where, there being sensual desire present within, a monk discerns that 'There is sensual desire present within me.' Or, there being no sensual desire present within, he discerns that 'There is no sensual desire present within me.' He discerns how there is the arising of unarisen sensual desire. And he discerns how there is the abandoning of sensual desire once it has arisen. And he discerns how there is no future arising of sensual desire that has been abandoned. (The same formula is repeated for the remaining hindrances: ill will, sloth & drowsiness, restlessness & anxiety, and uncertainty.)

https://www.accesstoinsight.org/tipitaka/mn/mn.010.than.html

To self-chose the right "place to work", like a costumer selects his/her food according to his/her desires wouldn't work for a lay person (one not in main training yet).

That being the reason it's tradition to devotef and faithful approach a teacher, to take refuge and silas, after an outwardly sacrifice (usually candles and incense sticks) and then ask for ones "working place".

Don't think that one could meet a teacher in home and fastfood enviroments, it required to access borderlands: What is a boarderland, how to recognice?.

It might be that what ever is given as task, wouldn't seem proper. nevertheless, one should follow it eager and with faith. One might know such circumstances from movies like "Karate Kid".

If thinking "smart" and trying to gain things on short cuts, avoiding sacrifices required, full of stinginess, it will not work how ever much info and intellectual access there might be and one would just nurish hindrences instead.

[Note: This gift of Dhamma is not thought for wordily gains, exchanges, stacks or what ever trade. In the case the places does not provide for liberating shares, it may be deleted best]

Furthermore, meditation often has some other object of focus (namely, the breath, the movement of the abdomen etc.) does making the five hindrances the working-ground change this?

The five hindrances are only 'working ground" when they exist. When they don't exist, the breathing becomes the working ground. Or if the five hindrances are sufficiently weak so they can be calmed by awareness of breathing; the breathing is the working ground.

should we try to categorize the thought into one of the hindrances, then accept it merely as a thought, as part of a process which does not constitute our identity and then watch it impassively until it fades?

Yes, this method can be used. However, personally, I think it does not optimize the development of wisdom.

Or how should we deal them?

The suttas report Buddha categorised thoughts as "skilful/wholesome" & "unskilful/unwholesome" (refer to MN 19). This categorization was based on the wisdom that such thoughts are either harmless or harmful. If the mind truly penetrates the harm or potential harm of unskilful thoughts, they are easier to eradicate.

For example, most people don't crave the drug heroin or get angry at their boss. This is because they know clearly the drug heroin leads to serious problems or their boss can fire them from their job.

There is not much to say about it, the sutta named Subhasutta (numbered MN 99) has plenty of times the word kammatthana -- also translated "occupation" in this translation.

Here is the definition from the famous ATI glossary:

kammatthana Literally, "basis of work" or "place of work". The word refers to the "occupation" of a meditating monk: namely, the contemplation of certain meditation themes by which the forces of defilement (kilesa), craving (tanha), and ignorance (avijja) may be uprooted from the mind. In the ordination procedure, every new monk is taught five basic kammatthana that form the basis for contemplation of the body: hair of the head (kesa), hair of the body (loma), nails (nakha), teeth (danta), and skin (taco). By extension, the kammatthana include all the forty classical meditation themes. Although every meditator may be said to engage in kammatthana, the term is most often used to identify the particular Thai forest tradition lineage that was founded by Phra Ajaan Mun and Phra Ajaan Sao.

Anyway, the end of your question is more important -- i.e. you asked:

More practically, if a thought arises which is unwholesome (not during a sit, but in everyday life, since it is stated above that we should make the "structure of [our] entire life" based on the working-ground) should we try to categorize the thought into one of the hindrances, then accept it merely as a thought, as part of a process which does not constitute our identity and then watch it impassively until it fades? Or how should we deal them?

Dividing your life into two parts, "trying to to get the citta into samadhi" and "not trying to get the citta into samadhi" is not a good idea, even for people who are not bikkhus (mnks). What you can do in daily life is sati sampajanna (clear and constant awareness), like when you walk, when you eat and so on -- but getting citta (mind) into the samma samadhi (right concentration) needs seclusion, which is exactly what samma samadhi is.

So first a bit of vocabulary...

For the buddha, "living in the present" means having the citta (mind) in samadhi (concentration) of the first jhana (or higher) as expressed here, and it turns out that "the here and now" is always pleasant,

so,

• the citta is in the samadhi of the first jhana = living in the pleasing here and now
• the citta is not in the samadhi of any jhana = not living in the present moment.

The buddha also says that it is easy for puthujjanas (wordly people -- who probably have some incorrect views e.g. about the doctrine) to confuse the samadhi of the jhanas with Sallekha (meaning "effacement").

The puthujjanas, who say that "watching thoughts come and go" is "living in the present moment", say that because watching sensuality is all they know -- and they do not know how to experience something else than sensuality. Those puthujjanas are full of thoughts 24/7, so they say that when they watch thoughts they "watch reality".

What the puthujjanas who invented dry meditation (i.e. "meditation without concentration") do is, at best (and it is really at best), is sampajanna (clear awareness) -- since they do not have sati (mindfulness or awareness or concentration), it's not really sati·sampajanna (concentrated or continuous clear awareness):

"And what is the development of concentration that, when developed & pursued, leads to mindfulness & alertness? There is the case where feelings are known to the monk as they arise, known as they persist, known as they subside. Perceptions are known to him as they arise, known as they persist, known as they subside. Thoughts are known to him as they arise, known as they persist, known as they subside. This is the development of concentration that, when developed & pursued, leads to mindfulness & alertness.

meaning they watch vedana-sanna (feelings and perceptions), and thoughts come and go, and they try to pass that as yoniso manasikhara (clear seeing r insight).

"And what is the development of concentration that, when developed & pursued, leads to the ending of the effluents? There is the case where a monk remains focused on arising & falling away with reference to the five clinging-aggregates: 'Such is form, such its origination, such its passing away. Such is feeling, such its origination, such its passing away. Such is perception, such its origination, such its passing away. Such are fabrications, such their origination, such their passing away. Such is consciousness, such its origination, such its disappearance.' This is the development of concentration that, when developed & pursued, leads to the ending of the effluents. http://www.buddha-vacana.org/sutta/anguttara/04/an04-041.html

There are even toxic puthujjanas who hype the "acceptance of any experience", "embrace the present moment" (which is, for those people, watching thoughts and sensations). Those puthujjanas fail to see that there are meritorious vedana-sanna, meritorious thoughts -- and conversely demeritorious vedanna-sanna, demeritorious thoughts -- and that letting demeritorious experience arise is very bad.

Sati·sampajanna is what people today call "mindfulness". It is good at preventing future lusts arise (or at least preventing them from becoming established), it is "what guards the senses" (see here and here -- but to suspend the bad feelings, you must get the citta into samadhi of at least first jhana, precisely because the vedanna-sanna of this samadhi is the meritorious vedana-sanna.

And to get the citta into samadhi, you have to memorize and put into practice right view and right intention, so first being physically secluded from other humans for a few hours or days -- then it is mano (mind) which has piti (joy associated with concentrative absorption) and the kaya (body) has passambhati (calm) then sukhaṃ vediyati (experienced ease), and the citta (mind) has sukhha (well-earned ease) then samadhi (concentration).

THe buddha explains how he got right samadhi here and here (renunciation) -- and there's a a page with a few quotes on nekhamma (renunciation) here.

Once he had right samadhi, the citta could know how kamma-jati (karma and birth) and dukkha (misfortune, dissatisfaction) works. So like the buddha said, sati·sampajanna is what he did first, judging his thoughts and classifying them into two (i.e. "meritorious" and "demeritorious"), destroying the bad thoughts with seeing the danger of bad thoughts, but it is precisely this sati·sampajanna (=situational wisdom, that is: In each moment one calls to mind whatever is necessary to master the given task ) which is transformed into samma samadhi. So do not divide your life into "meditation" and "non meditation". Later on, this samma smadhi is used to turn the citta towards āloka·saññaṃ manasi karoti (perception of light) and yoniso manasikhara (clear seeing).