TacticsMaster on F-Droid also says it has 100k puzzles, sourced from https://database.lichess.org/#puzzles, though that says it has 5 million puzzles, so perhaps 100k is just a coincidence, a nice number, or maybe of the 5m there's a nice subset.
> Generating these chess puzzles took more than 100 years of CPU time.
The page includes a link to the puzzle generator and tagger for interests sake.
To add to this thread, here's how Lichess generates chess puzzles - https://github.com/ornicar/lichess-puzzler