What first came to be known as "Rhythm & Blues" was actually in simple terms = uptempo Blues w/ a hard backbeat
Yes, early Rock n Roll music is simply Rhythm and Blues with a name change
Jazz is a swung music w/o backbeats (before Hardbop and Fusion came along via Black Gosepl and then Rock n Roll)
Country music is early blues + white anglo-celtic twang. Country didn't have any backbeats until it absorbed R&B and became Hillbilly Boogie/Rockabilly and modern commercial Country
No, it's really a SPECIFIC music that began in the 1940s/1950s
This is because FKA Twigs uses/sings in Soul music-derived style; In the beginning before it was called "Soul", the style of music that Ray Charles birthed was ALSO called R&B
Modern "White Rock N Roll" stayed closer to Black Electric (and acoustic) Blues (a variant of Rhythm & Blues that Chuck Berry and Bo Diddley came out of)
While Blacks acts mixed in Black Gospel vocals to the R&B (which is technically Rock n Roll under its original name) and turned it into Soul->Funk/Funk Rock->Disco
"R&B" isn't a catchall term in the way "Race music" was but obviously, it can be used in a way to segregate music/musicians.