Seating assignments at PDX union station for coast starlight

hi all! i’m taking the coast starlight taking off from PDX union station here pretty soon. it’ll be my first time on a train, and i’m a “know it all in advance” type of traveler to help curb my anxiety. having trouble figuring out how boarding and seating works as there doesn’t seem to be assigned seating of any kind. can anyone explain the boarding process to me from start to finish like i’m 5 years old?? does seating get assigned? can you sit anywhere and everywhere for the duration of the journey? any and all tips help. thank you in advance!🫶