I agree with just going for it when it comes to asking strangers for their portrait. If you’re friendly and confident (and know how to take rejection swiftly and just move on), you’ll have a 90%+ success rate. Having a weird old film camera bumps that up even more, in my experience. So does opening with a compliment (“oh wow I love your outfit”).
However, taking a portrait of someone without their consent in 2023 is just a dick move. This debate has been had so many times under so many angles that you just can’t claim ignorance anymore if you take yourself seriously as a photographer.
A great book I enjoyed recently is Dawoud Bey on Photographing People and Communities.
Depending on where you are in the world and whether you obviously fit in within the cultural fabric or not, people might get very offended/aggressive about it (you deserved it), or not say anything but that doesn’t make you any less of a dick.
Just ask. It can be as simple as pointing to your camera and showing a thumbs up with a quizzical look waiting for a nod in busy situations, it’ll take a second. If the person is performing/selling/etc, buying something/leaving a tip beforehand will help a lot.
If asking scares you so much, that picture isn’t yours to take.