"B Movie" is from the days of double-features under the studio system. The B picture was the picture other than the main attraction, the A movie... tended to be more cheaply-made and more spectacle-oriented, more exploitation-like... not unlike the serials that also were often packaged with the double-features.
Many were produced by so-called poverty-row studios like Monogram and PRC...
You can download many, many traditional B-movies from the Internet Archive... archive.org...
I suppose that after the demise of the studio system and the rise of the drive-in era, the term came to include independently-produced movies of exploitation content that came to be screened at the drive-ins. Some were produced by companies like American-International, many directed by "king of the Bs" Roger Corman... So many who worked for Corman ended up as important big studio directors in Hollywood, and Bs and serials were the templates for the big summer movies, the escapist movies that became so big in the late 1970s onwards...
So B movies have a long and honourable history in Hollywood and much is owed to the Bs and those who made them...