Ethereum

A truly private l2 that's been under construction for almost a decade now

Launching with stage 2 decentralization on day 1 with full onchain (opt in) privacy

In other words, a zk rollup that actually uses the zk property of zero knowledge and not just succintness

So are all transactions private or just anonymous? Where can I read about it?

If you hold your secret key you can see it. The privacy is opt in, so you can also choose to hold certain amounts of certain tokens as "public balance" and rest as "private balance"