Entrance beyond Mexican border "limits" and their "Free Zones" are for 4 days maximum, seldom checked unless something is going wrong. After, or Beyond, requires the FMM card for Canadians and Americans, and if you are driving, a deposit on the value of your vehicle, RVs excepted, as they require something else, called a ten year permit. The FMM is not truly a visa, as one does not apply for it, with subsequent checks before it is issued; a visa is what Canada required of Mexican nationals about three years ago for the first time, and why, no one knows. This created quite a stir in these two countries, both bordering on the USA, and having many of the same issues because of that.
Good question, what happens if you do not have the FMM card. It is said they will want you to pay for it (sometimes a separate step after you present your passport to acquire it) and carry it with you at all times when you are inside the country. Likely what will transpire, is that you will never be asked for it, but if you have any sort of accident or altercation, and fail to be able to produce one, it becomes an added reason to detain you, and a fine will be levied.
If you fly into Mexico you will get the same card, usually paid for with the price of your ticket. The FMM is good for 180 days, but you will give it up if you're returning by air.
Remember that if you need transportation by air out of the country (or across any international border) you will require a passport, any other form of pass, will not be accepted.