I had installed Projectionist long time ago, but did not manage to learn enough about the promises seen in help - it seemed a bit too succinct.
Your Finding Files Semantically and other Projectionist related parts are great, the book is worth even it includes only these parts.
One of problems I had was, that commands
:Eresource etc. were not known to neovim and I had no clue, what was going on.
Finally I have discovered, that my
.projections.json was broken - one of keys (file name pattern) was present twice.
After reformatting the file, the duplication got removed and things started working.
I would propose to mention a hint: if things are not working with Projectionist, check the
.projections.json files and start with linting. May be you have some other tricks for that.