Covij=BiBjσm2 market model how to get that

I have a question

Covij=BiBjσm2 is the equition of market model in notes. I tried to find where it derive from.

since Bi= ρim*σi/σm, the equition can be Covij=ρim*σi*ρjm*σj. But this conflict with Covij=ρij*σi*σj.

Want to know why and where am I wrong. Thank you!

the market model used regression

Does that mean this formula is specific for market model?

Any logic behind the formula then?