The foundation is footers, piers, block, tie-downs, and anchors built to the home's installation manual or an engineered design, and the county inspects it. Skirting is the enclosure added after setup. It is not the foundation and it must not hide unfinished foundation work.
Short answer: Foundation requirements come from the installation manual or an engineered design for the specific home and site — pier spacing, footings, anchoring, and for a double wide the marriage line supports — and the setup inspection checks them. Skirting requirements are about ventilation, access, materials, and grade, and skirting goes on after the foundation and utility work are inspected.
Reviewed by a manufactured home setup contractor and dealer.