what's the actual way to get string in minecraft?

piglin bartering, if you're in the nether anyway. throw gold ingots at them and string turns up a lot. not worth doing for string alone, but if you're bartering for pearls or fire res you end up with a pile of it as a side effect.

Spiders are the obvious one, they drop up to 2 each. But the real answer is cobwebs. Break one with a sword and you get string (break it with shears and you get the cobweb itself instead, which catches people out). Abandoned mineshafts are full of them, you can walk out with a stack in ten minutes. Cats also leave string as morning gifts sometimes, and you can fish it up, but neither is quick.