For DML (INSERT, UPDATE, DELETE, MERGE) log is continuously applied, and isolation levels are remapped to row-versioning-based isolation levels on readable secondaries:
Read-only workloads for disk-based tables use row versioning to remove blocking contention on the secondary databases. All queries that run against the secondary databases are automatically mapped to snapshot isolation transaction level, even when other transaction isolation levels are explicitly set. Also, all locking hints are ignored. This eliminates reader/writer contention.
Offload read-only workload to secondary replica of an Always On availability group
However the system catalog views can never be read using row-versioning, so the application of DDL (ALTER TABLE, TRUNCATE TABLE, DROP TABLE, etc) changes to the secondary by the REDO thread still requires an Exclusive Modification Schema Lock (Sch-M) has to be coordinated with the readers who all acquire Schema Stability Locks (Sch-S). So long-running queries on the secondary can block the redo thread if DDL changes are pending to an object involved in the long-running queries.
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On the secondary replica, the read-only queries acquire schema stability (Sch-S) locks. These Sch-S locks can block the redo thread from acquiring schema modification (Sch-M) locks to make any DDL changes. A blocked redo thread cannot apply log records until it is unblocked.