I only see them at work

This is going to vary widely by the type of person you are, the type of people at your work, and your work culture. Some coworkers go out to happy hour and parties together, and some have no engagement with coworkers outside work. As long as you're not being toxic or causing problems and stay professional at work, you can set your own boundaries where you want to.

I don't lay a boundary, I just tend to be very slow to trust. I've made three incredibly close friends from work but I never specifically sought them out.

at the door?

While I have work friends, and we do go out and drink, it's almost always within or adjacent to the context of a work event. Like maybe we're doing a co-workers happy hour. Or we had some work function that stretched into the evening, so we'll go out for dinner and drinks together afterwards. There's definitely been a few times where we were drinking for like 8hrs after a late lunch! But it's very rare for me to hang out with them outside of that. Like I can count on one hand the times I met up with co-workers on like a weekend just to head to a bar.

As far as topics, they're still friends, so we talk about all sorts of stuff. But there's definitely more of a focus on things happening at work and less about our private lives, especially spouses/SOs. My current "work best friend" will sometimes call me afterhours and we'll chat and vent about stress at work and stuff, and even some of our stresses at home/personal lives, but the latter tends to be more surface level. With my non-work friends, we'd certainly get deeper into those topics if desired.

It comes down to professionalism. That's really the "firewall" between my work friends and non-work friends. Yes my work friends and I can have fun, joke around, get a little loose when drinking, share deeper stuff about ourselves, but we never want to cross that line. Sometimes it's thin, and sometimes it's even a little blurry or dotted. But we all still strive to never (or very, very, very rarely) cross it.

Limit outside of work social interactions to going away, retirement, and holiday parties, maybe occasional mutual hobby stuff like bowling or sports; basically large group outings a-ok, 3 people gathering sparingly, 2 person stuff is a bit too intimate

I don't have a set boundary, I just have intense social anxiety so I have difficulty actually transitioning to the friends outside of work part. 99% of the time it doesn't happen even though I'd like it to. But I definitely have had my "at work" friends until either they or I have moved away.

My personal boundary is that I can bitch about work to them, but I keep it light when talking about my personal life. Trusting someone with heavy shit when I'm not offering them friendship feels wrong to me.

I'm sure my coworkers think I'm a robot but what else is new.