Do each airline has its own terminal

It changes depending on the flights. I saw two airlines with gates right next to each other

No

Concourse A, Main Terminal Seattle - Tacoma International Airport's concourse A (Main Terminal) serves American Airlines, ATA, China Airlines, Delta Airlines, Frontier Airlines, Hawaiians Airlines, and Sun Country Airlines. It contains gates A1 - A14. Concourse B, Main Terminal Seattle - Tacoma International Airport's Concourse B (Main Terminal) serves America West Airlines, Charter Airlines, Continental Airlines, Horizon Air, and Southwest Airlines. It contains gates B1 - B15. Concourse C, Main Terminal Seattle-Tacoma International Airport Concourse C (Main Terminal) serves Alaska Airlines, Charter Airlines, and Horizon Air. It contains gates C1 - C20. South Satellite Seattle-Tacoma International Airport South Satellite serves Aeroflot Airlines, Air Mobility Command, Asiana Airlines, British Airways, Charter Airlines, China Airlines, Eva Air, Northwest Airlines, Scandinavian Airlines, and World Airways North Satellite:  Air Canada, JetBlue Airways, United Airlines and US Airways.

check airport gate map to see where each airlines are located

I believe Chris Anderson has the correct information.