What about collecting model cars or keyrings, hobby or pastime? Collecting vintage cars?

I think anything that holds the potential for making money could be considered a hobby. A collection can be turned for a profit.

there are quite a few women in my brazilian jiu jitsu class, two purples and a few blues

I don't agree with this. Hobbies come down to what the individual considers as their hobby imo. In job interviews I'm usually asked what are my hobbies, and I don't give them a list of things that I would consider worthy of calling a hobby - I tell them how I spend my time - including what I'd consider are my hobbies.

Instagram could very well be a hobby - you're building a photo album, and developing photography skills - and you're putting it out there to be judged by the world. 99% of instagram I would consider vapid slooting selfies, but that doesn't mean instagram can't be a hobby.

To me, a hobby is just something that interests you, and some way to spend your time that most people aren't doing. If there's a subject, and/or a skill, I'd say it could easily be called a hobby.

I wouldn't consider watching tv a hobby because most people watch tv, but if you were into war documentaries or certain genres or something, I could see someone considering it their hobby.

Working out of something I used to be consider my hobby because it was a bit more niche at the time, and it involved a lot of researching into the subject, and applying that information into diet and routines. I no longer consider it a hobby because it's just a routine for me - no more goals, no more PR's, not even planned sets - I just do whatever while I'm there for my allotted time to maintain what I have. But if someone else considers it their hobby, then it's their hobby.
Keeping a beta in a fish bowl most wouldn't call their hobby, but there is a lot to delve into when getting into aquariums: water chemistry, bio-diversity, breeding, beauty/aesthetic in recreating nature through aquascaping. Growing aquatic plants is a whole nother level too - balancing lighting, nutrients and co2 is a lot more involved than with terrestrial plants.

I know someone who likes to look at rocks. Not even collect the rocks - just look at them. But he knows how to differentiate between all of the different types, and plans his hikes based on the geology of the area - so subduction zones have more metamorphic rocks and possibility of jade or serpentine - that kind of thing. Pretty interesting actually.

That's more of a passion or life mission.

You don't need to explain. We all know this. Just go out and see for yourself the majority of sports and hobbies are dominated by men. Sloots like her reflect their own perceptions onto others. There's no debate about it to be honest. I'm not mocking women, it's the nature of men to craft, build, and explore.

like some ******* before me said. Women are wired to desire to have children and care for them, they don't find material things and objects as interesting as us men do.

They're not into STEM pursuits, but obviously they are VERY much into material things (especially if they connotate social status).