Do I need to trigger my ESTA?

You seem to be quite confused about the differences between ESTA and the VWP, as well as rules for Mexico and the United States.

ESTA is just a pre-authorization which allows you to board a flight bound for the US. It does not grant you entry into the US (actually even a visa doesn’t). It is valid for 2 years, but just lets you board the plane. What seems to be confusing you is that you don’t need an ESTA if you enter the US by land, but that does not change the rules for the maximum duration of your stay.m, which are governed by the VWP, not ESTA.

The VWP (visa waiver program) is the scheme under which you will be allowed to stay in the US without having asked for a visa beforehand. It will allow you to stay for 90 days, and that does not change based on whether you arrive directly from the UK or enter by land from Mexico.

What may be adding yet more confusion is the fact that when you are in the US under the VWP, the 90 days are not reset when you go to neighbouring countries and islands and come back. But they still start at the time you first enter the US, whether you come from one of those neighbouring countries or not. It’s not like the US+neighbouring countries were a single travel area with no internal checks like the Schengen Area in Europe is (or is supposed to be). The US will still check your admissibility whenever you arrive, whether you arrive from Mexico or from the UK, by land or by air.

If you want to spend more than 90 days in the US, you just can’t do it under the VWP. You will need to get an actual visa, the specifics of which are dependent on your situation (what you intend to do in the US, how long you want to stay, your situation back home, etc.). Getting a visa will require you to submit an application, pay fees, go through an interview, etc.

The ESTA isn't a visa. It's just an electronic document that pre-clears you, in a sense, to be admitted to the US - a heads-up, if you will. It's only required on flights and ship voyages (except ferries between Canada and the US in B.C./Washington).

You still are subject to the terms of the Visa Waiver Program, and an I-94 visitor record is still required, so entering the US by land, sea or air doesn't change how long you can stay.

Much of the confusion here seems to be related to your status in Mexico rather than your status in the US. For example, you wrote in a comment

Once the American 90 days have started I can't go back to mexico for another 180 days.

Actually, you can. Mexico does not care about your US status under the VWP, and Mexico allows you in for however long it allows you in regardless of whether you've recently been in the US or are coming from the US. What you can't do is get back into the United States after doing that. And actually, as we'll see in a bit, you might even be able to do that.

going to mexico from US comes under the same visa as me coming to just the US.

Only as far as the US is concerned. That is, your time in Mexico counts against your authorized period of stay in the US. But Mexico doesn't care about that in the least. They will authorize you to stay in Mexico independently of your VWP stay in the US.

The rule we're struggling with here is designed to prevent people from doing visa runs from the US into Canada, Mexico, or an "adjacent island." The US does not want you to spend 90 days in the US, pop over the border for a couple of days, and then return to the US for another 90 days. But you seem to want to do the opposite.

The rule has an exception for people who actually reside in one of those places, but the rule is also worded in such a way that border officers have discretion to invoke the rule or not:

An alien admitted to the United States under this part may be readmitted to the United States after a departure to foreign contiguous territory or adjacent island for the balance of his or her original Visa Waiver Pilot Program admission period if he or she is otherwise admissible and meets all the conditions of this part with the exception of arrival on a signatory carrier.

Source: 8 CFR 217.3(b).

So if you try to reenter the US more than 90 days after your first entry, the rule does not apply to you. Suppose you spent a week in the US, go to Mexico, and then come back the day before your original 90-day period ends asking to stay for another week. The officer "may" readmit you for one day, but may also decide to give you a new 90-day period.

Now, because this rule is designed to prevent border runs, and it concerns only Mexico, Canada, and "adjacent islands," you might wonder what would happen if you spent 90 days in the US, then went to Costa Rica for a couple of days, and then came back to the US. In fact, you would be likely to be refused entry anyway, because the immigration officer has discretion to do that. Even though you went to Costa Rica, you would nonetheless appear to be abusing the VWP.

Now, to your specific comments:

Now I have a large chunk of money. I want to go to mexico for a year or how ever long the money lasts. Plenty of people do it.

Instead of strategizing visa runs, you might want to consider a temporary resident visa, with which you can get a resident card:

This is a single entry Visa and will allow the applicants to enter to the country to exchange it for a Temporary Resident Card within 30 days upon their arrival at the nearest migration office (INM). The temporary resident card will be valid for one year and multiple entries.

You also wrote:

Once the American 90 days have started I can't go back to mexico for another 180 days.

As noted above, this isn't true. What you can't do is go back to the US during the initial 90-day period and stay past the end of the initial 90-day period. But if you just want to stay in Mexico without going back to the US, or indeed visit any country other than the US and return to Mexico, that's perfectly fine.

Now I've read a few posts that if you go out by land it's a different visa and the 90 days arnt triggered. Because I'm not going through customs?

Those posts are almost certainly mistaken. When you enter the US by land, you do not need ESTA, but you would still be entering under the Visa Waiver Program. If you follow the US news at all, you will be well aware that there are very stringent immigration controls for people crossing into the US from Mexico by land; there's no way you can do that legally without being inspected by an immigration officer.

In closing, let me assume that your goal is to spend roughly a year in Mexico with a visa run that includes visiting your family in North Carolina. In that case, you have two options:

1) Get a Mexican temporary resident visa. If you do that, your return to Mexico will reset the 90-day clock on the US visa waiver. For more information about that, you should ask at Expatriates.

2) Do the visa run to the US and then stay out of the US for the next 90 days. This assumes that Mexico won't take exception to your second 180-day entry coming so closely after your first; I do not know whether that assumption is at all reasonable because I have no experience with nor much knowledge of Mexican immigration practices.

You are confused about VWP and ESTA are and how they work.

As a VWP-country national, you can visit the US on the Visa Waiver Program (VWP), and you will be admitted for 90 days at entry. That's 90 days starting with, and including, the day you entered. It doesn't matter how you enter the US, by land, sea, or air; the conditions of VWP are the same. Leaving the US and re-entering on VWP will generally not get you a new 90-day admission period if you only go to Canada, Mexico, or the Caribbean islands.

ESTA is a travel authorization, valid for 2 years, to allow you to board a plane or boat if you wish to enter the US on VWP by air or sea, but not needed if you travel by land. Whether you have an ESTA or not (if you enter by land) doesn't affect the conditions of VWP, including the 90-day admission period.