Here's a micro crochet Belle in her blue peasant dress and apron. She has a tiny petticoat, and a hair-bow which can be seen here: [link] and was crocheted with single strand DMC cotton and an 0.75 mm crochet hook.
To be honest, I saw some teeny tiny micro-crochet animals on etsy and thought, I could do that. It turns out I couldn't, but then I crocheted a little wee person, and she turned into my ballerina, and I just... kept going. I had already learned to crochet in the round making a number of my larger dolls, so going smaller wasn't actually that hard.
The only thing i've made with thread are cross-stitch pictures and bracelets with cool designs and some with names and symbols. I'd love to learn to crochet so I could attempt something like this. I'm sure my kids would love to have them as little toys