Blocking on readable secondary replica
Queries against read-only secondary replicas implicitly run in snapshot isolation regardless of the session isolation level or RCSI setting. This avoids blocking due to DML changes. Read-only queries still acquire schema stability locks, which will block DDL operations by redo threads and visa-versa.

SPID 129 blocks a session ID 45 ( i am sure this is not a user id) for
almost 6 hours which is dependent on spid129 and wait type is
LCK_M_SCH_M

In your case, it seems a redo thread is waiting for a schema modification lock but is blocked by a very long-running query/transaction schema stability lock. Other queries are in turn blocked by the redo thread.
Take a look at the SELECT query execution plan and duration of the transaction for remediation.