US customs zone only covers the 50 states, DC, and Puerto Rico. All other territories have their own customs agencies and jurisdictions. USCBP does not (mostly) perform customs functions in Guam or the CNMI.

When you transit Guam between US flights, you won't pass through Guam customs. When you arrive into SPN, you'll pass through CNMI customs.

Flying SPN-GUM, USCBP inspects passengers for Guam admissibility in the gate area. If you enter Guam, you go through Guam customs. If you connect elsewhere, there's no inspection.

Flying GUM-HNL, USCBP inspects passengers for Stateside admissibility before boarding. You can call this an immigration inspection, but it's not the same as an international flight. Then when passengers arrive HNL, their routing skips immigration and goes directly to baggage claim, then USCBP inspects your belongings for customs purposes.

The Guam/CNMI visa waiver program has a few extra countries/territories, and CNMI has a separate labor visa program leftover from its immigration-autonomous decades pre-2009, and an additional visitor parole arrangement not applicable to Guam.

Since everyone admissible to Stateside is admissible to Guam/CNMI/USVI, and everyone admissible to Guam is admissible to the CNMI (but not vice versa), flying HNL-GUM-SPN you won't see USCBP, but on the reverse direction you will see them up to 3 times.