For submarine cables, there aren't that many strands, so getting it right is probably worth it, for terrestrial fiber, there may be 100+ strands, so they usually can sort things out at the ends, and I think you just fuse any strand to any strand. Sometimes, only some strands break and you can reallocate strands by priority until the cable can be fixed.

Strands in a bundle would likely have color coded sheaths. As long as you're not colorblind, it's pretty straightforward matching what gets spliced where.