The Fjordur map of Ark Survival Evolved has 7 bosses. The main world boss can be summoned at any of the obelisks, but all the others have specific terminals you use to teleport to the boss arenas.
Two of the boss terminals are not found on the main Fjordur map, so you will have to teleport to different realms. To do that you can travel to the Portal Cave 40.3 57.6 and use the terminals there, or on PC you can hold down the R key until a menu wheel appears and select that. Steinbjorn is located in Jodunheim, which is extremely cold. Only with high-end fur armor will you survive more than a few minutes in that realm.
All of the boss terminals are located in caves, with the exception of the terminal for Hati and Skoll, which is out in the open in Asgard.
Fjordur Boss Terminal Coordinates
Beyla 4.4 47.3
Hati and Skull 20.5 37.1 in Asgard Realm
Steinbjorn 77.6 30.8 Jotunheim Realm
Megapithicus 56.8 84.8
Broodmother 57.2 65.9
Dragon 86.2 5.0