In that example, it should have been first-hand or firsthand. When multiple words are used together as an adjective, in this case an adjective-noun pair, those two should become hyphenated. I just wrote another example – it’s ‘adjective-noun’ because the two words are used as an adjective for pair, even though they would not ordinarily be hyphenated.

For the same reason, asking “is it ‘first hand’ or ‘first-hand’” would be meaningless, because it depends on the context, unless a stylistic choice is made to write ‘firsthand’ instead

I’m in my mid 20s and a woman and I found out recently that the skinny jeans I’ve been wearing for the last decade have supposedly been uncool the whole time. I don’t care. Style is personal, and I like them.

Maximum dynamic pressure. At max q, the mechanical stress on a spacecraft stops increasing and starts decreasing. If a spacecraft is going to blow up, it usually does so before max q.

Not sure how to make that apply here 🧐

Even if you want to separate sex and gender and define sex using sexual characteristics (not actually a good idea, see works by Judith Butler and Julia Serano among others, although I wont fight that point here), almost no sexual characteristics are immutable. The only ones that I can think of are chromosomes and gametes, but chromosomes aren’t even binary (or observable without a microscope) and gametes arent a good basis either – should being infertile affect your sex?