i heard a rumor few years ago that some reissues were just using MP3 files as the source
is there any truth in this really?

the original press of a CD or vinyl would be from a master disc yes?

so if a new record label is reissuing the album 20 years on how is it done
are they given a copy of the master to press for themselves?

or are they given some high quality file and told to make their own master?
or are they just given permission to RIP a CD and then make copies

would be disappointed to think the best source possible wasn't used

you're answering your own questun right there, the best possible source is usually used, but the original master tape is not always available/in existence, so they have to make do with what's the next available (best) thing, including those options you mention...

there's some rare cases, where, for example, some family member owns the master tape and they don't reach an agreement with the label/surviving band members, so the latter use a second source, or where the master tape is (considered) "lost", then resurfaces for a later reissue, or where a label are dumb f**ks (no names mentioned), but as a general rule they want to sell the best possible product too

useful tip: you can make your own remasters at home, with the best available source (to you), and some music editing software... no more sleepless nights over a too dynamic release from the 60s