Hey ndisg -- unfortunately there is no handbook I can pass on to you on how to be a woman, even though it feels like everyone seems to know how to do it. You're totally right, gender is a social construct - meaning, we are not born with gender but are taught gender through what we experience through interpersonal relationships, media, communities, society at large, policies, etc. Often, our first "teachers" are our parents through their words and actions. If you're feeling some disconnect and your mom is from a different country, this might mean you didn't have what is considered "American femininity" modelled for you. If you haven't had many other female friendships, this might mean you haven't been able to learn from there. However! I want to emphasize this is not a failure on your part, your mom's part, and any other person who feels this way's part. As you already identified, there is no correct or right way to be a gender. Here is a resource
I think might help you understand this a little bit more. I would like to add too that there's possibly another layer added to this related to race and nationality, as white femininity has been historically held up as a standard due to colonization and white supremacy.
As much as I can sit here and tell you that gender isn't real, there are still very real consequences and emotions that come with feeling like how you do. I want to ask you though - are you interested in these expressions of femininity because you want to be or you feel like you need to be? In what ways are your experiences different than those you're thinking might be standard?
Regarding your specific questions:
I think you answered your own question about about what all-female friend groups do -- every person is different and thus every group is different. I kept trying to think of activities to mention here, but as soon as I wrote something I thought of many people I knew who didn't do that. I know this is sort of a non-answer, but the most common activities in female friendships are likely emotional support and enjoying each other's company. I imagine it's similar to any other friendships. If there were common activities, what do you think they'd be if you had to guess? This is not a test by any means, I'm wondering what kind of messages you might see or feel about this.
There are some women who shave their legs every day, there are some who shave some days, and there are people who don't shave at all. It might feel like other women have shaven legs every day because they might actually. American society usually sends messages to us that clean shaven legs are feminine. I actually don't shave mine at all. I have experienced times where that has made people around me uncomfortable, but it doesn't bother me and it's how I express my gender. Despite what you make feel like you should be doing, it comes down to what feels best for you. Despite the messages you may receive from the world around you, there's no wrong way to do this.
With bras, the first question I want to ask is do you want to wear one or do you feel like you should be wearing one? I'm sensing that you feel a lot of pressure here. There are plenty of people who have breasts that choose not wear one, and I want to remind that it's an option for you and you can choose that based on... anything. Like how it feels? Great! Wear it! Hate how it feels? Great! Don't wear it! On to your question, the best way to determine your bra size is to be measured, whether at a store by a trained person or at home with a measuring tape. Not all store that sell bras will have employees who measure (for example, Target or Walmart) but a department store with a lingerie section (for example, Khol's) or a specialty store (like a store that specifically sells bras/underwear) often will. If you'd like to try it at home, here's a guide
that I think is helpful. Personally, I think it can take some trial and error. Sometimes you measure one way and the bra you pick doesn't feel right. Only goes to show you... it's hard to generalize the experience of wearing a bra.
I hope this helps!