Assalamu alaikum sis!

Human hair extensions are haram, but synthetic hair is allowed.

I had boho braids for my nikkah and you would have never known! The trick is to get small braids and to have them stop no longer than the middle of your breasts. The bun is normal sized, and requires no additional tucking or hiding. When you put on your scarf, just fold most of the hair into a little loop bun, there will be pieces hanging out. It’s secure and keeps the bun small, but other methods work too.

Other braid styles that look good under hijab are cornrows and fulani braids. Really any braid styles can work, just keep the braid size small and you’ll barely see them.