I was pretty disconnected from my body growing up. I found sports embarrassing because I was overwhelmed by all the action and didn’t respond quickly enough. As an adult, I have taken an interest in fitness and it’s something I rather like now. I do tend to be thin and look more toned and shapely now. I workout 3-5x/week and easily block out other people now. I’m thinking the point here is that tert Si Types may come to overcompensate for obliviousness or awkwardness in these body awareness areas.
As for public....yes, sweats are not an option for me. It’s not about appropriateness (which I have a personal resentment towards at time...it might even be love/hate since I can sympathize with the reasons at times)...it really stems from vanity and image consciousness. I started cultivating a personal style when I hit puberty. Before then I was less aware of myself. By high school, I had a rep for being stylish in an avant garde way and almost something of a trendsetter, which is odd because I was also a loner and a honors student dork. But people always liked my style

. As an adult, I have a pretty solid conception of what suits me but like novelty, so I’ll be aware of trends but selective about what I use for myself. Appropriateness can still be a challenge for me as it feels so arbitrary at times.
Anyhow, I was always creative and arty since I was a kid, but I didn’t apply it to my appearance until a preteen. I also experience(d) it as a form of self expression (probably more feeling related there), and it’s a way of having a voice and asserting an identity when I otherwise feel invisible or forgettable. So some motivation is the pure aesthetics and my personality. I’m not particularly unique, but it’s about what suits me and embodies my mood.
I identify more with the IEI in socionics though. This resonates a lot more without me for Si in MBTI....people often attribute sentimentality to INFPs because of tertiary Si, but that’s not really me.