Upgrade from 4.2.10 to 4.2.12

I ran a second composer update, from nothing works here anymore

What did you get exactly?

What do you mean by "nothing works"?

I ran composer update and went from version 4.2.10 to version 4.2.12.
Once this was done I looked at what the changes were in the two versions, reading the documentation I checked and updated the files that have undergone changes in the project space.
As reported in the documentation, most of them were just formatting or changes you have comments, among these files the composer.json file is also listed, so I also updated this and subsequently re-ran composer update, after this I find a blank page and the files in the vendor folder are different and as a result nothing works.
I then tried to instantiate a new structure with composer, just to see if the composer.json file had changed, and it actually remained what it was before.

So, either I didn't understand, and therefore the composer.json file shouldn't be updated with the one in the vendor folder, as for all the other files listed, or I don't know.
Among other things, now I see that there is a new update with several files to be updated outside the system folder