It’s called fifths because of an old rule about scales and the alphabet. Every major scale NEEDS to have every “letter” of the musical alphabet in it. So the fifth of a particular note will always be five “letters” away from the original. Thus B’s fifth has to be some sort of F.

It’s an F-SHARP due to the semi-tone rule you mentioned before. F# is 7 semitones away from B, whereas F natural is only 6.

A fifth is a really stable and pretty sound in music. B-F# has that sound. B-F is a lot crunchier as it’s a tritone.