How does one view the terminal (tty) output externally? (Not a new ssh session, but what is currently going on)

How does one view the terminal (tty) output externally? (Not a new ssh
session, but what is currently going on)

Basically I've set up a few Raspberry Pis running different programmes,
and I'd like to see what is being outputted on them. I can obviously
connect via SSH, but that's a new tty session. Tried googling it, but I
think my terminology is a little odd!