Even though the resource database is read-only, it is updated by SQL Server, right?
No. It contains no data that would require modifying, just the definitions of many of the system objects
Does sys.dm_db_missing_index_details have information that results in "missing indexes" from ad hoc queries that have been ran once?
Maybe. It keeps only a certain number of index suggestions.
sys.dm_db_missing_index_details resides in the resource database, right?
The definition might, the data most certainly does not. It's a DMV which means it's a view into an internal memory structure, it's not data stored in a database on disk.
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