Hi bbuck002,
All mattresses need some type of support system underneath them (usually either a box spring, a foundation, or a platform bed) but which type is most suitable for a particular mattress depends on the type and design of the mattress. There is more in post #1 here and the two posts it links to in the second paragraph about the types of support systems that generally work best with different types of mattresses.
If you are building a bedframe that has a rigid non flexing slat surface with narrow gaps that were suitable for the mattress you choose then you wouldn’t need a foundation on top of it since it will perform the functions of a bedframe and a rigid non flexing foundation.
Some types of mattresses (mostly innerspring mattresses with coils that are joined together with helicals) may still do better with a functioning box spring that has some flex but the manufacturer of the mattress will be the best source of information about the support system that would work best for the design goals of the specific mattress you choose or that would meet the criteria of their warranty because each manufacturer may have different warranty requirements.
Phoenix