system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do they remaster really old movies and make them HD/Blu-Ray Quality?

All searches have since given very minimal information on the subject, how can they truly be HD when remastered (To be fair, the remastered ones do look REALLY good)
gpt
They usually start by going back to the original recording, and scan them into digital format.  This means they get higher resolution than available VHS or even cinema copies, and eliminate any copy errors or degradation.

They then pass the video through various filters to improve color balance and sharpness, and possibly touch up or fix a few scenes.