Is writing Author A & Author B the same as writing Author A, Author B in the author list? (when publishing a paper)
You don't have a choice, the journal's style guide will do it for you. In cases where there is a list and not a block (or otherwise separated), an ampersand will be used for the last author in every case I'm aware of.
In citation formats I'm familiar with (e.g. APA), using a comma with only two authors is never correct.
In other words, Author A., Author B. (2020) looks like a mistake, as if the third author (who would be separated by an &) were missing.
It makes no difference with regards to who is the "first" author.