score:4
Short answer: typically no but sometimes yes. It depends on the route + your vaccination status + flight date. No need for SEA-SAN.
Long answer:
See https://www.delta.com/us/en/travel-planning-center/know-before-you-go/travel-requirements-guide
U.S. citizens and residents traveling between destinations within the 48 contiguous United States and Alaska are not subject to entry requirements (Hawaii, Puerto Rico, and the U.S. Virgin Islands may have requirements). Plan your next domestic trip now using ourβ―Delta Discover Map.
The Delta Discover Map is convenient to see the COVID test requirements (and it doesn't mention the "U.S. citizens and residents" criterion), e.g. no need for COVID test for an SFO/LAX/SAN->SEA or SEA->SFO/LAX/SAN flight:
but one needs for a COVID test for a SEA->HNL flight if before March 27 + not vaccinated.