Grafana used to be so simple, I don't know if I'm a fan of this direction towards many services.
Having to run alertmanager and configure it in addition to Grafana was bad enough, now you need to run and configure another service if you want some extra functionalities for those alerts? Are they going to keep maintaining acknowledgements and scheduled silence in AlertManager now that OnCall exists? Are we going to have "legacy notifications" in AlertManager when not running OnCall, the same way there are "legacy alerts" in Grafana when updating from Grafana 7 (pre-AlertManager)?
AlertManager does not do the evaluations, it does not connect to any metrics database; those are done by Prometheus/etc and forwarded to AlertManager, which handles deduplication and routing among other things.
A quick look at OnCall suggests it is more for managing fired alerts than firing alerts.
Their own screenshot has AlertManager as an alert source.