How many losses does a team need before bookmakers consider it an underdog?

[Player101]

I often see people call a team an “underdog” just because they’re on a losing streak, but I’m not sure that’s how bookmakers think. I’m actually interested in knowing what formula bookmakers use to define it. Also, I’d like to know how they handle situations when two underdogs play against each other... like how do they calculate odds?
(I believe this kind of question can be valuable for everyone, both beginners and experts)

[Blackjax]

Most sports have a table or a ranking system so just by looking at it you can see which team is an underdog.

When two underdogs play each other the odds are usually same like 1.8 vs 1.8 or one team might have a slight advantage like 1.8 vs 1.9.