They use GPU based enclaves. They have a white paper available. I just seemed it but they mention AMD and NVIDIA enclaves.

They can’t get your username from your phone number (but they can b’get your phone number from your username)

AFAIK it’s just a webapp.

Unblock signal first?

Signal also has been a leader when it comes to encryption and metadata protection. Their cryptographic protocols are public domain and are the gold standard for private messaging from pretty much every perspective other than decentralisation. (it’s still centralised)

Signal is American but run by a non profit so it can’t be sold and is open source under GPL3 without a CLA from what I can see so it’s unlikely they would ever go closed source.