MacBook Pros -- since the first "retina" models released in 2012 -- have a sprayed-on anti-glare coating.
I'm going to guess that the coating consists of extremely fine particles combined with some kind of adhesive to bond them to the surface of the display panel, but that's just A GUESS.
In any case...
... Since the beginning, this anti-glare coating has proven to be quite "fragile" -- that is to say, it doesn't take much abuse before it begins to degrade and "flake off".
Hence, the condition the OP mentions.
It's called "StainGate".
Once it starts, it's "non-fixable".
There is no "repair", other than replacing the entire panel.
What can cause it...
..."Over-cleaning" the display. Numerous posts in this forum over the years by folks complaining about StainGate, who also mention that they are careful about "cleaning" the display.
It's actually "the cleaning" that is DAMAGING the sprayed-on coating and grinding it off !!!
The best way to maintain a MacBook Pro display is to clean it AS LITTLE AS POSSIBLE.
Don't "get it dirty" in the first place.
When there is dust on the display, don't "wipe" it (by "wipe" I mean use pressure).
Instead, "dust" a cloth across it -- NO pressure.
If there's something that dusting won't clean off, use a cloth moistened with water and as little pressure as possible -- then dust dry.
DON'T use stuff like alcohol-based cleaners, etc.
OP:
Be aware that you aren't going to "fix" a used MBP that has StainGate (at least a fix that restores the anti-glare coating).
The only way to "get rid" of the blotches and stains is to COMPLETELY REMOVE the anti-glare coating. There are videos to watch on YouTube about how to do this.
You'll be left with a "glossy" display, but "the stains" will be gone.