Where is the most concrete exposition on the Buddhist ethics in the source material?

The Dhammapada is a popular and famous summary (here in the West at least). It more or less opens with "non-hatred" (aka "loving-kindness") and has other "ethical" chapters. It's better for your purpose to read the verse[s] only, not the stories which accompany each verse.

I think the "most concrete" exposition of Christian ethics is the Great Commandment (in two parts) -- the Sermon on the Mount is ... longer, and maybe more fanciful. Anyway there's one verse in the Dhamapada which (for what it's worth) seems to me very close to the second part of that, i.e.,

129. All tremble at violence; all fear death. Putting oneself in the place of another, one should not kill nor cause another to kill.

(which may seem to conflict with the anatta doctrine; "but there you go" as is said colloquially).

Furthermore:

• There's one sutta (which I remember as, "the sutta of the four directions") which contains a great deal of mundane practical advice for lay people¹

• There are the four "brahmaviharas", which these articles describe e.g. as follows ...

  These four attitudes are said to be excellent or sublime because they are the right or ideal way of conduct towards living beings (sattesu samma patipatti). They provide, in fact, the answer to all situations arising from social contact. They are the great removers of tension, the great peace-makers in social conflict, and the great healers of wounds suffered in the struggle of existence. They level social barriers, build harmonious communities, awaken slumbering magnanimity long forgotten, revive joy and hope long abandoned, and promote human brotherhood against the forces of egotism.

  The Brahma-viharas are incompatible with a hating state of mind ...

• The primary ethical 'laws' are the five precepts.

• There's maybe a gradual training for laypeople -- virtues includes "harmlessness" and "giving (dana)".

• There are three (sometimes two) root poisons.

• There's a list of hindrances (a bit analogous to what some Christian theology calls "sins") -- but there are more less detailed lists of these (and from different schools), see also especially fetters for example.

It's hard to summarise in one place² -- I'd be remiss not to add that it says a good friend is important (to some extent all-important) -- but then again there's "seclusion" (and so on).

¹ _{There's at least one whole paperback of advice to laypeople, based on i.e. summarising suttas like this one -- almost all of which is about the "virtue" (perhaps aka "ethics") part of the threefold training -- e.g. if you count advice like this as part of "ethics".}

² _{possibly because Why isn't there a Buddhist Bible? to which the accepted answer doesn't even mention Mahayana.}

Buddhist objective is nibbāna, the safest.

It's like the science's, engineering's or computer programming's rule, we can't misunderstand at any level of the process. It's buggy, risk and unsafe.

Therefore, it's very hard to understand Tipitaka and Atthakatha as alike as science's, engineering's or computer programming's rule.

So, if you point your expectation in Buddhism only at the easy part, you will miss the safest part, which elaborated multi aspects of reasons. That missing is buggy, risk and unsafe.

So, concrete exposition of Buddhism is the conclusion like:

1. Vinaya is in VN Parivāra by upāli, the vinaya secretary of buddha.
2. Sutta is in KN Paṭisambhidāmagga by sāriputta, the stand-in teacher of buddha in MN Mahāgosiṅgasālasutta (or alternative in translated Visuddhimagga but if you bias of the supernatural, it's not your choice).

Both above require these to understand:

1. Pāli skill from Kaccāyana.
2. Tipitaka analysis skill from Netti.
3. The discipline of everything from abhidhamma by sāriputta, the stand-in teacher of buddha in MN Mahāgosiṅgasālasutta. Or, saṅgaha by anuruddha, past laṅkā's pontiff.

But if you want to start in Buddhism, you need:

1. Qualified teacher to teach you step by step.
2. Easy sutta to recite and do follow, such as Maṅgalasutta (and all Khuddakapātha), Siṅgālakasutta, and dhammapatha.

The Buddha taught for 45 years therefore his teachings are infinitely larger than those of Jesus. Therefore, there are so many (reported) discourses on ethics. Below are merely a few examples:

• Magga-vibhanga Sutta: Noble Eightfold Path (for monks)

• Cunda Kammaraputta Sutta: Ten Wholesome Actions (for laypeople)

• Veḷudvāreyya Sutta: The Golden Rule (for laypeople)

• Sigalovada Sutta: The Layperson's Code of Discipline (in detail)

• Vanijja Sutta: Right Livelihood (for laypeople)

• Samajivina Sutta: Marriage (for laypeople)

• Karaniya Metta Sutta: The Buddha's Words on Loving-Kindness

The above said, a fitting equivalent to the Sermon on the Mount, in terms of both content & style, is the Maha-mangala Sutta, which similarly is about "Blessings".

In addition, as the Sermon on the Mount was spoken 500 years after the Buddha, most of the topics in it would be found in the Buddha's teachings, such as:

• Integrity & Dhamma having the taste of salt

• Anger

• Lust and sense control

• Retaliation & Love Your Enemies

• Spiritual treasure

• Don't Judge Others

• Crossing the floods with unshakable faith.

As for divorce, the Buddha taught life-long marriage is the ideal (Samajivina Sutta) yet it appears did not prohibit divorce when a wife is immoral.