What are the names of the tables in the Nakama schema? (specifically the user added collections) I don’t see it in the storage engine API.
The storage engine API is built on top of the storage table in the Nakama schema. We use specific indexes and take advantage of btree indexes to provide fantastic performance on the single table design. And wherever we need to achieve greater scale on the size of the table we use logical “sharding” with PG11 table partitioning on top of the storage table. Although we’ve only needed to do this on one game title so far. 
You can find the Nakama schema here. Although a quick summation is visible with the \d psql command:
List of relations
Schema | Name | Type
--------+--------------------+-------
public | group_edge | table
public | groups | table
public | leaderboard | table
public | leaderboard_record | table
public | message | table
public | migration_info | table
public | notification | table
public | storage | table
public | user_device | table
public | user_edge | table
public | user_tombstone | table
public | users | table
public | wallet_ledger | table
Is there any extensions or sample products that extend Store and Catalog type functionality on top of Nakama?
This is a great suggestion. We’ve not got any open-source code to use as a reference for a virtual store/catalog and inventory design but I’ll take it to the team and see how quickly we could put an example together.
I’m not sure I can answer some of these because I don’t have the information yet myself.
No worries. Based on the info you do have to hand I think that it’d be quick and simple to structure the virtual store and inventory on top of the storage engine API.