Presumably after the fact by linking them entirely within Jira. You could mention the commit in the ticket, but that wouldn't make a back-link from the commit if someone's browsing from that direction.

That said, I don't really mind mentioning the ticket in the commit. It's relevant to what one is doing when one makes the commit.

But then I use gerrit at my job, so I don't have room to complain about others' systems.

I think naming the branch and/or the pr title after the jira ticket works too. I think it just needs to contain it, too. My feature branches usually look something like feature/proj-123-short-description. But to be honest, my extended commit descriptions usually contain the ticket number too, so I may be remembering wrong.

Not sure if that's better or worse, though.