The memory-optimised tables are memory-resident, they cannot not be in memory. What memory is left over from that is the normal SQL buffer pool and managed as normal for the buffer pool. If the memory-optimised tables grow, the buffer pool shrinks.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability