You take short fibers, and twist and splice them together to get longer fibers. And you repeat until you get the desired length, or thickness...

Lots of videos of how to make it on the Internets using nettle, or some other plant (flax is still used to make clothes today though through industry methods).

It’s the trick where people would take two phone books and if you interleave every page you can literally have two cars going each direction and can’t rip them apart. Ropes are this on a smaller scale.

The fibers don't need to be long. About 1 inch is enough for any purpose. The trick is to twist each fiber individually in one direction (fe. clockwise) while twisting them around each other in the opposite direction (fe. counter-clockwise). This creates internal tension that makes the fibers push against each other with substantial force, creating sufficient internal friction for them to bind against each other. And now you have string, and you again switch twist direction to bind multiple strings together to make thicker string, a switch again to bind multiple of those together, etc, until you get to proper rope thickness, which is basically always substantially thicker than the length of the fibers that make it.

It's a bit magical when you figure out how to do it, it doesn't seem to make any sense yet it works.

Thanks everyone for the explanation!