Let's say I have three tables A, B, and C. 

- Table A is a domain table (lookup table). 
- Table B is a many-to-many mapping table. 
- Table C is a regular table, containing a fuzzy matched entity. 

When creating/editing a new record for Table C, I want to do an insert into Table B using a known ID from Table A, and the new ID from Table C. 

The reason for this is because Table A is a type table and Table C can have multiple simutaneous types from Table A mapped to it.

In the stepper for editing Table C you will include a Collection Step on the entity B. Once you have that, then you're able to add in the multiple simultaneous relationship types that you want.

But the key additional point that you may not have in mind is that there's a tab with explicit features to make it easier to handle many-to-many relationships. Tick the "Many to Many Relation" field, and you'll find that xDM makes it really easy to pick from the many values in Table A that you want linked to your new item in Table C.