I’ve quoted the relevant code section below. I’m designing a home using a slab on grade per the IRC2021 code.
Am I understanding correctly?
1. I cannot use frost protected shallow foundations for that home’s unheated garage or porches. These areas must still extend below the frost line. But these deeper foundations can indeed be structurally connected to the shallow foundation of the frost protected slab on grade.
edit – I’m wrong about #1. There are 2 design approaches available, one for heated structures and one for unheated structures. I can use the latter. Someone else explained to me that the second approach can also be applied to the entire house for consistency, if desired.

2. Under the main body of the house I cannot mix foundation types, such as slab on grade in one area with crawlspace in another, unless that crawlspace is also part of the conditioned space. Ditto with basements.
edit – I’m still not sure about this one. But I wouldn’t be doing a ventilated crawlspace anyway. So it’s not relevant.

IRC2021
R403.3 Foundations protected from frost… shall not be used in unheated spaces such as porches, utility rooms, garages and carports, and shall not be attached to basements or crawlspaces that are not maintained at a minimum monthly mean temperature of 64 degrees F.