Interesting thank you

theres a ratio that certain friend chats will switch gp from rs3 to osrs for a fee

This.
Or you could sell your rs3 items/gp on that section and buy osrs gp. There are also legit websites that will transfer etc.

Osrs but only in appearance because everyone's on 5 accounts and there's 500 bots on each world

Osrs has over 2x the playerbase... I remember someone in a YouTube video mentioning that, however I dont know if that includes bots.
Also as someone who has played both quite a but, in my opinion oldschool makes so much more sense on mobile than rs3