While @xLeitix's answer pretty much sums it up, I would like to add that
"A clever person solves a problem. A wise person avoids it."
Students complain about marks if either
a. Your rubrics/grading criteria are unclear or inconsistently applied
b. The students perceive you as pliant.
Of course, there are those students who will just complain for complaining's sake or with the hopes of you just not wanting the hassle of dealing with them. Those are, in my experience, a very small (albeit vocal) minority. Moreover, since their complaints are rarely justified, you can easily handle them.
If you handle (a) well, then students are much less likely to complain about you, and would greatly appreciate the transparency. I make my rubrics clear-cut and public, and ask students to refer to specific rubric items in their regrade requests. I tell them in advance that I do not grade feelings or intentions, just what actually made it to the assessment paper. Make sure that this grading policy is public on week 1; perhaps even get students to acknowledge reading it (say via an online form).
Students won't perceive you as pliant if you fix (a) and stick to it. I actively encourage students who threaten me with going to a higher authority to follow through on their threat (politely), something along the lines of
"You are well within your rights to take this matter up with the dean, and if you feel like you have been mistreated in any way, then by all means do so".
No one has gone on to complain yet. They know they have no standing.
Given that we are teaching in unusual times, you can (and should!) be lenient when the situation warrants it, as long as you are consistent about this as well. If one student complains about an inconsistency or possible interpretation and you find their argument valid, go back and fix it for everyone. If a student asks for an extension for no particular reason, grant it for everyone.