Version control - division of git multi-branch functions, and operations after merge

Question

Maybe the title description is not very clear

For example, I have a project now, the main branch is master, and then there are two additional branches A and B (ignore other branches)

A developed some functions and then merged them into the master.
Then B developed some functions and merged them into the master.
(-> means merge to)
If there is a very strict functional division, for example, the functions of A and B are completely parallel and do not affect each other,
then only A->master, B-> ;Master has done this, right? (I don’t know if this is the process for regular development)

If A is a public lib and B is some functional development, then what should be the merge process?
A->master, master->B, then B completes the development and then B-> ;master

Or A->B, A->master, B->master

Please answer

Answer 1

A->B, A->master, B->master