SQL Server - One of the cores always at 100%
could be a few things, start by checking the wait types.
A couple I have seen
RSS Receive Side Scaling is not enabled so all n/w traffic is handled by a single CPU
or
Ghost cleanup running - this is a system SPID responsible for removing deleted records and runs on a single core
https://docs.microsoft.com/en-us/sql/relational-databases/ghost-record-cleanup-process-guide?view=sql-server-ver15