What does mean IOT structure? Actually, this is Index Organised Table. Some databases like Oracle has really good implementation this type of table for end user.
To be shortest, IOT table stores whole data in BTree index.
Please take a look on high level comparison how to work simple Heap (Standard) Table with Btree index and IOT table.
So, as you can see everything in one BTree index.
Nail had really research project and realised from scratch IOT DB. Of course will a lot of improvements , but initial kernel of database has been constructed by him. Just take a look on comparison benchmarks between PostgreSQL and IOT databases.