Where can I find information about the maximum current density for each metal layer?

Tech lef has dc current density for EM check, is that what you are looking for?

Yes, in fact :)

Were you able to find the tech file or need help with finding it?

I didn't look yet, as I'm busy with something else. But if you could tell me the path to this tech file I'd be thankful :)

where is it located?

DCCURRENTDENSITY https://github.com/google/skywater-pdk-libs-sky130_fd_sc_hd/blob/ac7fb61f06e6470b94e8afdf7c25268f62fbd7b1/tech/sky130_fd_sc_hd.tlef

Do you know if this information exists in documentation (other than this techfile)? Thanks!

I could not find it in the documentation, but since it's skywater authored I would assume it's correct.