Your substructure and platform can be one in the same or can take the shape of two separate constructs. Over the years, we have seen multiple iterations of this but they all have 3 things in common:
If you build a circular platform and substructure and plan to lay decking over top, it is advised to space your joists the same distance apart as a square deck (12”-16” on centre). This will allow for decking without the need for additional support.
In the past, we have had customers make their substructure into a deck and use that as their platform. This does make it more difficult to install a rim and decreases your floors insulation capacity. This is a better option for warmer climates. You can screw your rim boards directly to the decking, or support them through the use of L brackets.