For software where we're upstream, e.g. Calls, Chatty, kernel, phosh, phoc, squeekboard, ... it would be best to file issues at the upstream repositories (either at source.puri.sm or gitlab.gnome.org ).
There is also also the more general "OS-issues" repository here:
https://source.puri.sm/Librem5/OS-issues/
So it depends a bit on what you have issues with. If you use matrix you can DM your username and you can get invited to the #community/librem5 channel.