Hi Mario, and welcome to the PID community!
I think you might find some of our introduction to PIDs resources in the Knowledge Hub helpful - see Getting Started with PIDs - The PID Forum for example.
Basically, PIDs pretty much break down into three broad categories
- PIDs for people eg ORCID and, to a lesser extent, ISNI, as you say
- PIDs for places eg ROR (the Research Organization Registry and Ringgold, a proprietary organizational identifier)
- PIDs for things - the biggest category as this can include any type of output (articles, books, datasets, etc - Crossref and DataCite DOIs are among the most widely used), research objects (eg IGSNs for geological samples, RRIDs - research resource identifiers, etc), grants (Crossref DOIs), and more
You might also find this article by @TomDemeranville (ORCID’s product director) helpful as it talks about what makes a “good” persistent identifier, including why PIDs should resolve.
Hope this helps!