How to use project.el

[excalamus]

With 27.1 out, I see talk of project.el, how great it is, and how it's been 
around for years.  What I don't see is anything about how to use it.  There's 
one sparse page in the manual, C-h i m emacs m Projects.  I've run 
project-find-file and project-search.  I fail to see how they're fundamentally 
different than find-file or grep.

I happen to have ESS installed which appears to use project.el.  It defines 
some new methods.  I'll need to brush up on CL objects and generic methods, 
though, before I can decipher how those interact with project.el.  In the 
meantime, does anyone have a basic setup and usage example?