What is the “suggestd” process in Activity Monitor?

Normally it is related to Siri suggestions

Aside - user profile is Mac mini, macOS 11.3 Big Sur with is outdated as Big Sur is presently listed as Big Sur 11.6.1 and brings bug and security fixes.

Welcome and thank you too.

All processes ending in the d does not necessarily mean it is associated to Siri. There hundreds if not thousands of native Apple Processing ending with the d at the end.

To get an idea open Activity Monitor >> View >> View All Processes. Not every D ending process belongs to Siri.