ACINQ pioneered the lightning service provider (LSP) model. When you install Phoenix Wallet you are installing a lightning node on your phone. It is not a full bitcoin node. It's not even a full lightning node. Instead, it delegates all of the heavy lifting to the ACINQ service. Your node opens one channel to the ACINQ node and relies on ACINQ to handle lightning routing, liquidity, splicing, and a bunch of other stuff that requires an always-online service. In return, you pay fees to cover the cost of these services.

ACINQ deployed the first mobile lightning wallet and have been at the leading edge of mobile development for ~7 years now, by my count. They are the unsung heroes of lightning, IMO. I'm a huge Phoenix stan.

They usually blog their biggest engineering achievements. They don't post often but when they do it's a banger. Highly recommended reading! https://acinq.co/blog