I fiddled with the seat/handlebar adjustments on my 29er MTB until I think I got it right. Your down knee should almost be straight when pedaling, and you should not be leaning forward too much on your wrists/shoulders. I had to get a new adjustable riser bar for my handlebars that raised the bars by about 3", and relieved some shoulder ache that came from longer rides. It also took care of my hands falling asleep.
I see so many adults on a bike that is too small, with the seat too low tend to that grumble about burning knees. BTW, the seat should be tilted forward a bit to get the full effects of the padding.
My hard, narrow, barely padded seat was immediately replaced by a Bell "Soft-tec" seat with room for the manhood, and suspended with extra cushion, and springs.
https://www.amazon.com/Bell-Comfort-Soft-Tech-Bike/dp/B0791CL5DT