re: the Beatles' tape loops, they were just that,... tape loops.
They were short, spliced segments of tape, of which the beginning and end were taped together, forming a loop. Then, they'd put the tape loops on R/R tape recorders, in play mode, and bring these signals into the mix with a mixer. "Tomorrow Never Knows".
Tape loop is a reel-to-reel concept, and technically, you can't do that on a 414, or any cassette. There are things you can do with samplers and computer software that emulates 'tape loops'.
There's a whole musical genre based on electronically generated loops, but I guess that's another post.