So it seems like Buster comes default with 2.7 on load and an alternate 3.7 version of Python. However, I am using this Raspberry Pi 3 B+ just for an application that is only compatible with Python 3.5. How do run Python 3.5 as default version or remove 3.7 entirely from Buster?
just need to follow 1 step
do not remove python3.7 directly it will cause problems 
install 3.5 and directly start using it.
after 3.5 install you can remove python 3.7
keep it simple. :)
Downgrade Python 3.7 to 3.5 on Raspbian Buster