Bonus question: There is a tiny sliver of class D airspace surrounding
the surface B of SeaTac on the western, southern and southeastern
parts of the bravo that is surface. Anyone know if this belongs to
SeaTac because there is no other field in the center of the area and
if so, why on earth does a bravo airport also have a tiny section of
class D surrounding it?
In the FAA's "Airspace Designations and Reporting Points" document, this airspace is specifically described as "Class D airspace... extending upward from
the surface and designated as an extension to a Class B surface area." (page D-164). It is the only example of this kind of airspace in the entire US national airspace system.
There is also one single example of "Class D airspace... extending upward from
the surface and designated as an extension to a Class C surface area." It is at Biggs Army Air Field at El Paso, TX (KBIF) (page D-165).
The purpose of the Class D "extension" at SeaTac is to require approaching pilots to communicate with the control tower, without imposing the requirement for an actual clearance to enter the airspace, and without imposing the other requirements associated with Class B airspace. Yes, it does "belong" to SeaTac.
The Class D "extension" at KBIF is more of a "hybrid" situation-- KBIF has its own control tower but is actually located in the Class C surface area of El Paso International (KELP). The Class D "extension" is intended to facilitate operations at KBIF, and abuts the Class C surface area of KELP.
On a related note, Los Angeles International Airport (KLAX) has Class D airspace associated with it that appears to be functionally equivalent to the Class D airspace "extensions" at SeaTac, but it is not described as an "extension" in the "Airspace Designations and Reporting Points" document. Instead, it is described on pages D-148 through D-149, in the main section on Class D airspace, section 5000, whose description (page D-1) includes the phrase "Each Class D airspace area designated for an airport in this subpart contains at least one primary airport around which the airspace is designated." Each piece of Class D airspace at KLAX is described as part of a ring of Class D airspace that completely encircles KLAX. There are two such rings in total, one extending to 2700' AGL and one extending to 2500' AGL. Most of the Class D airspace in each ring is invisible and non-functional because it coincides with Class B airspace, leaving just two protruding "tabs" of functional Class D airspace that serve essentially the same purpose as the Class D "extensions" at SeaTac.
Page number references are to the FAA's "Airspace Designations and Reporting Points" document, current edition Order JO 7400.11D.