Listening to the greats of the generation

You wrote that a Mera Da'Tara has authority over the community that accepted him and those who belong to it. The question is whether I am necessarily associated with a community if I was born into one community or another, or perhaps communal affiliation is determined by a person's desire to belong to a particular community?
Also, I understood that Rabbi Yosef Karo has the status of a Mera Da'Tara in the Land of Israel. The question is whether this means that everyone who lives in the Land of Israel is bound by his rulings?

It is customary today (unlike in the past) that a person belongs to a community according to his origin (=his father), and from there he takes his customs. When it comes to a community, this is certainly the result of a choice and decision to belong to it. There are several communities everywhere, and therefore the location does not determine.
The claim that Rabbi Yosef Karo is a Mera Da'Atra here is unfounded. Although Rabbi Ovadia used to say this, no one is really excited about it, and rightly so. There is no such thing as a Mera Da'Atra who is dead. I have never heard of this strange concept.

For the halakhic origin of the term ‘Gedol Hador’, see A'amo'r's book, Part 1’ Page 84 – Retrieved here: http://www.eretzhemdah.org/data/Uploadedfiles/ftpuserfiles/books/government_and_country/1.pdf

Regarding ‘Mara Da'tra Mat’ – the meaning is that there are places that have taken upon themselves the rulings of a scholar even after his death. See here:
Responsa Rashba Part 1, Mark 1
And from this path, all those who used to do all their actions according to one of the great rabbis in the place where they used to do all their actions according to the laws of Rabbi Alfasi, may his memory be blessed, and in places where they used to do all their actions according to the group of the Rambal, may God bless him, and indeed these great ones did as their rabbi did. And who is it, if there is one wise and worthy of teaching and sees evidence to prohibit what they permit, that he practices prohibition. These are not as their rabbi in the place of their rabbi, if they do not do according to his words, they will belittle the honor of their rabbi in his place.

When I said there is no source, I meant a primary source. Regarding the renunciation of the Jews, there is a source (at least for the methods that refer to the greatest Jews of the generation). But I do not know of a source for a general authority for the greatest of the generation. Indeed, some of the first wrote this without a source, but the things are puzzling. Where does this authority come from? After all, the authority to renounce wealth proves the opposite, that they received authority for this very reason and not for all the other powers of the Sanhedrin. Otherwise, what did the Sanhedrin hate? And the matters of education are known but puzzling and not agreed upon. Perhaps the authority is, as the Rabbis say, the consent of all Israel, which is the authority of the Sanhedrin, but then again we are talking about consent and not authority. And when there is no consent, there is no authority. However, if there was a halachic authority for the greatest of the generation, it does not depend on consent (just as the authority of the Sanhedrin does not depend on consent). And the point is that even if you hold that someone is the greatest of the generation, you have no obligation to listen to him, and this is the situation in our time with regard to those who are accepted as the greatest of the generation. There are also some of the first who wrote that the greatest of the generation can inflict corporal punishment, but this too has no source. This is simply the necessity of the hour (in their opinion) and not a law that was learned from any source. And regarding a place where they used to do as one Posk said, the words of Responsa Zichron Yosef (Levan HaRassh, also cited in Kosh 2b, I believe) are known, who wrote that this has no validity (at least in the place of Mera Da'Atra, as you also cited here from the Rashba). In any case, we are not talking about a Da'Atra because he was the Rabbi here and has authority, but rather this is at most a custom they accepted to act according to his opinion (and not an obligation by virtue of authority). And the point is that the mouth that forbade (=the public) is also the one that permits, as far as a situation of halakhic authority is concerned. In any case, here in Israel this is not the case. Even in the time of Rabbi Caro, they did not act here according to his opinion.

“The claim that Rabbi Yosef Karo is a Mera Datra here is unfounded.”

But elsewhere you wrote that when I have no position in halakhah I must follow Rabbi Yosef Karo. Why can't I follow whoever I want?

Because the custom is to follow it.

And what is the source of the fact that custom is what is binding?

You ask where one should follow customs? In the Pesht of the Gemara, there is a place where those who used to learn this from “do not leave out,” and some who learned this from vows. You can find reviews online.

So perhaps we should say that we should follow the example of these great men of the generation, since that is the custom?

This is not the custom. Some people mistakenly think it is obligatory, when it is not. And some do not do it at all.

Rabbi Elchanan Wasserman, in his book Divrei Chachamim, wrote to prove that today, if the majority of the great men of the generation rule on a certain ruling, there is an obligation to obey them, as in the great court in Jerusalem (Medina, do not deviate)