Author here, and happy that you're interested in the software itself rather than the implementation =)

I'll probably write a detailed post about file indexing, since it's pretty interesting from a technical standpoint.

I looked into spotlight (via `mdfind`) but it was too slow and I wanted to port to Windows/Linux later, so your speculation is correct: Finda has its own file walking and indexing mechanism.

It walks a user-configurable set of directories, but not in the background --- it's actually done every time you open Finda, so you only pay the CPU cost when you are actually using the program.

The index is stored in memory, and size is proportional to the number of files/folders walked. However, Finda does respect .gitignore files, so you can exclude things you don't want to show up in the search results (or consume memory).