I found this quick glossary helpful when I first started educating myself about these issues:

Privilege is about access. If you're born into wealth and have good connections, you can get into a lot of spaces other people would work hard to even be considered for. In many cases it's enough to just "look" the part, i.e. being white and male opens many doors (even if you aren't even aware of those doors because you didn't know they are barred for others).

Underrepresentation is about numbers. Representation alone is not obviously important, but being underrepresented almost every time can affect how a group is perceived. Representation "normalises" people, especially members of groups you might not think about even if there are many of them. E.g. if x% of people are non-heterosexual but far less than x% of movie characters are, and stigma or social exclusion means you barely ever see any actual people like that either, you'll think the actual percentage is far lower.

Marginalisation is about treatment. It often goes hand-in-hand with underrepresentation because minority groups tend to be mistreated more than if they are in the majority. This doesn't have to be actual physical violence or direct insults, it can also simply be a lack of consideration: if an event is planned by young fit men they might think chairs are unnecessary although it excludes people who need to sit down occasionally for health reasons.

Diversity is about variety. It's not nearly as important that your event includes "one of everything" as that it doesn't just consist of variations on the same theme (e.g. white 20-something guy from Eastern Europe, white 50-something guy from Western Europe, white 20-something guy from the US). White men shouldn't be your default -- in fact you should try not to have a default at all. Try to have a mix, the more varied the better. Having a "default" leads to everything else being somewhat of an "other" and thus easier to dismiss (leading to marginalisation). And even if an event doesn't already include people from a particular group they might feel safer to join if there's a healthy mix of different people from all walks of life than if it's dominated by a single group.

Inclusion is about experience. In other words, even if you invite a black woman speaker to your all-white mostly-male event, nobody will benefit if she's just a token. Diversity (i.e. making "othering" more difficult by making the groups more balanced) can help achieving this but for smaller events inclusion alone can already be a good start: if your all-male monthly meetup has a woman attending and she feels included (rather than just being "the woman" or having to try to "fit in") that might lead to more women participating.

So when I say "marginalised groups" I mean not simply underrepresented groups but groups the tech community treats poorly. A lot of spaces in tech are very unwelcoming if not actively hostile to women.

This isn't a new thing, I experienced it even when I grew up in the 90s although as a boy I didn't think of it that way at the time: it was "normal" that women were underrepresented, so that clearly meant women weren't interested and any woman showing interest was either not really a woman (because "there are no women on the Internet") or not really interested (because "everybody knows women aren't interested in tech"). This behavior results in women being excluded even when they want to participate and in being forced to "play along" (i.e. prove "they're not like the other girls").

You're right that there are many multiply-marginalised people. This is what intersectionality acknowledges: a black woman is treated differently from someone who's just black or just a woman.

But the solution isn't to just shrug it off and decide that nothing can be done. It's still about real people suffering and limiting that suffering is a worthwhile goal even if you think it's unrealistic that it can ever be abolished completely.

Picking one WILL result in a fairer society. Because as soon as you normalise women in tech spaces, you can focus on something else. Tech is no different from any other part of society. You will screw up, everybody does, but it's important that you simply own your mistakes and learn from them and then try better next time. Yes, people will shout at you for anything you got wrong, but that just means there's room for improvement. They may have a point, though, if you're willing to listen.

Feminism has achieved a lot, especially for white women (and men, btw -- "toxic masculinity" is not about cancelling manhood but about calling out shitty behavior people previously got to excuse with "boys will be boys" or absurd standards of "manliness"). Many places including the US have made broad strides towards equality for gay and lesbian people. It's hard work and progress is slow and incremental (with the occasional leap in between), but there's definite progress.

EDIT: I guess downvoting counts as an argument these days.