> Are chiplets physically connected chips that come from different wafers?

Yes exactly. Take separate unpackaged chips, and put them all on one shared substrate (a "silicon interposer") that has wires to connect them. There are a bunch of different technologies to connect them. You can even stack dies.

https://www.imec-int.com/en/articles/chiplets-piecing-togeth...

I think typically you wouldn't stack logic dies due to power/cooling concerns (though you totally could). But you can definitely stack RAM (both SRAM and DRAM). Stacking is kind of a new process though as far as I understand it.