I’ll raise you with https://www.gmpuzzles.com/blog/tag/gas/ - lots and lots of human made sudokus and other puzzles. The link is for GAS (genuinely approachable sudokus)

Can you explain to me how the core backup process works? I browsed the git repo, but could not find it after about 10 minutes.

For example PostgreSQL: does it just do a pg_dump of a whole database? Or table for table? What about incremental backups? Are these implemented? And if so, what’s the concept behind it?

Otherwise I would not see any use for larger (dozens of gigabytes and more) databases.