What is needed to make a new app that connects and controls Raspberry Pie via bluetooth?

[Muzziball]

I need help to figure out what is needed to control the raspberry pi (3&4) from a custom made app via bluetooth rather than via (local)wifi and computer.
With information going both ways.

Its an exciting project/product that is already helping kids with autism, elderly with dementia and just in general making people want to move.
I will be happy to hear from you here or: [email protected]

[terribleted]

question has been asked before... here is one answer
viewtopic.php?t=245587