Or do big brother tell them when they need to.

Cause surely the housemates dont call Stephen Bear, Bear all day, and he same with Heavy D

they likely use their names - the ones they are known by... so if that is "biggins" or "bear" (which he has tattooed over his body then that is what they would answer to...

why not?

if their stage is 24/7 "cameras on in case something is said", they will be in stage mode all day...

we've heard lewis and marnie etc call bear "steve" several times. why do people need to try and find an issue with everything?

It depends. I've heard people call Bear 'Stephen' in there.

when the HLs aren't on, they refer to Lewis as "cockfeatures", just to break the ice