To a large extent, the pull request part of the story is irrlevant; we are really just talking about merging.
If two people work on the same file, their code might merge and it might not. Git thinks in terms of lines — which actually turns out to mean a line and its immediate neighborhood. In a 20-line file, if Alice edits line 1 and Bob edits line 20, that is mergeable without a conflict. If they both edit line 1, it isn't.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…