Your question is at risk of being closed for being unfocused, but I shall suggest regardless that these winged humanoids either have smaller and lighter bodies than you're probably imagining them having or have larger wings and bigger pectoral or whatever muscles than you're probably imagining them with. Nothing to do with mountain living, just to have flight be more plausible for them.

Do you want them to only live longer or have a way to deal with size problems(square cube law) and energy requirements issues as well? First is relatively easy and might not even need magic(except for giving them the natural qualities without evolution), second is going to require some magical kajiggering

Would muscles and a supporting skeletal system be able to handle the task of throwing a boulder though? With this answer I'm imagining a wide base and a tail-like prehensile appendage(whale-sized?) with a gripper at the end... not really sure if that would suffice, assuming the organism has the energy to function(is fed and has the heart to pump the blood to where it needs to be).

@PatrickArtner Whoops. Fixed. Thanks!

@Nosajimiki How would accelerated growth allow this? If you're saying they may be able to puppet the tree into flinging it, their control of an organism isn't like telekinetic force, but rather being akin to taking over control of their creature's muscles or, if the plant is capable of it like a venus fly trap, activating its 'hydraulics'

I guess this answers most of my future questions regarding biological sieging as well, even if I'm slightly saddened to already know siege towers won't work due to the SCL, and even if I may have to resort to ever so slightly equipping what might amount to a rhino/elephant hybrid with something that'd aid them in focusing their crashing through a gatehouse if chemically reactive 'breachers' aren't used, as was pointed out in William Walker's answer.