Much more than people expect, with one big distinction: 'iPhone not nearby' vs 'no iPhone at all'.
Without the iPhone nearby (but still paired): calls and messages work over Wi-Fi or cellular (with a cellular plan), music streaming works, plus Apple Pay, workouts, and maps — most core features. What silently needs the iPhone: app updates, initial setup of some features, and anything requiring the paired phone's connection when no Wi-Fi/cellular path exists.
Without owning an iPhone at all: that's Family Setup — an iPhone owner sets up the watch for a family member with their own Apple ID. It requires a cellular Apple Watch and a supported carrier plan. The family member gets calls, messages, location sharing, and activity tracking, but not health features like ECG, and the app experience is more limited.
For the most standalone experience: a cellular model with an active plan. Current watchOS 27 supports Series 9 and later, Ultra 2 and later, and SE 3. Wi-Fi-only models still handle calls and messages on known networks when the phone is elsewhere — for phone calls over Wi-Fi, make sure Wi-Fi Calling is enabled on the watch; otherwise try a FaceTime audio call.
Sources: https://support.apple.com/108300
— Osnivo Editorial