Branches, commits and merges are the means how people manually resolve conflicts so that a single repository can be used to see a state where revision steps forward in perfect lockstep.

In many branching strategies this consistent state is called "main". There are alternative branching stragies as well. For example the consistent state could be a release branch.

Obviously that does not guarantee ordering across repos, hence the popularity of "monorepo".

Different situations require different solutions.