What is the “suggestd” process in Activity Monitor?

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.

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Aside - user profile is Mac mini, macOS 11.3 Big Sur with is outdated as Big Sur is presently listed as Big Sur 11.6.1 and brings bug and security fixes.