I'm pretty sure everyone who cares about getting a good fit (and isn't simply trying the clothes on in person) is looking at measurements, which you can usually find for any half-decent vendor (though it may take some poking around their site). The best have it per-garment (or per-cut), less-good but usually still alright is having a guide to the measurements they base their sizing off of.
Even guys can't really get away with just "Small, Medium, Large" if they want a decent fit that they can predict from just the label. Modifiers for the cut become necessary (regular, slim, relaxed, extra-slim, that kind of thing). And that's for clothes that are pretty forgiving on the fit, like knits...
Women's clothes are even trickier. It's basically impossible to boil them down to one or even two size metrics or labels unless you're relying on a shitload of stretch in every other part of the garment, which is something that usually only very bad garments do (think: Temu). Women's proportions are also far more variable. Shoulder-bust-waist-hip often sees some pretty wild differences, like two women will match on a couple of those measurements and be way far apart on the others. Then you've got height to worry about. Dudes can be similarly far outside the norm of distributions for the relations between their key measurements, but it's not as common—most of us have it relatively easy.
Looking at the actual measurements, though, I've found to be very reliable. I buy almost all my clothes on eBay and directly from brands on their websites, with great success, because I know both my own key measurements, and the dimensions of clothes that fit me well (I have some notes, doesn't take a lot of data points to have enough to be pretty accurate). I've also ordered for my wife with a similar strategy, works well there, though you're way more likely to run into cases of "OK there are zero sizes of this garment that will work for you, just gotta give up on this one" because of the issue above.