“If a whole nation would be moved into my homeland, and from now on it wouldn’t be my homeland”
Yes, this is your ELI5. Majorities come and go. Governments come and go. I’ll give you two examples right next door to Israel:
Egypt was a dynastic system, then Geek, then Roman, then Christian nation then an Arab nation beginning in the 7th century. During each of the periods, a particular ethnicity did exactly the above: they moved in and became the majority. There was a point where it was overwhelmingly correct to call Egypt any one of the above after dynastic rule concluded. Today, Egypt is a Muslim majority country, but if for some reason christians poured in (the British kindof started to do this in the early 19th-20th century in the protectorate period) it would, at some point, become christian.
Constantinople was a Christian capitol city for centuries until the Ottoman conquest in 1453. The city was renamed Instanbul in 1928, but wasn’t recognized as such until a year later in 1929.
The takeaway from all this is that land changes hands in various ways. It’s the point at which the definition of a land changes that is sometimes controversial until a kind of revolution takes place.