They usually all have pager duty integration as well.
Some examples:
Datadog: https://docs.datadoghq.com/synthetics/ Grafana cloud: https://grafana.com/grafana/plugins/grafana-synthetic-monito...
1. Yes, a synthetic check is very useful here to just see if a user facing "thing" is still working.
2. a heartbeat check / deadman's switch can also work here, but it will only be reliable when the monitored event has a predefined cadence, e.g. "every 5 minutes this should happen".
3. The lack / absence of metrics flowing into a system is also sign. This would typically be solved by the Signoz team where they would alert on not seeing some specific event happening for x time. This can be tricky if the event is directly related to a user interaction.
Super big disclaimer: founder at a monitoring company that solve 1 and 2, not 3.