It’s common but phones like the 10R don’t have any support for unlocking bootloaders
It used to but oxygen OS is pretty far from stock now.
That depends on the model. Plenty of newer OnePlus phones use mediatech processors with little to no custom ROM support.
A fact I found after buying a new OnePlus phone without doing any research because I was in a desperate situation.
It’s common but phones like the 10R don’t have any support for unlocking bootloaders