There are 4 fossils.
Drake- One can be obtained in Dusty Bowl and Stow-On-Side. Repeatable methods are the Digging Duo. Refer to page (1) for more info. The finite methods are Shield only.
Dino- One can be found on Route 6 and Dusty Bowl in Sword only. Repeatable methods being the Digging Duo.
Bird- One can be found on Route 6 and Dusty Bowl in Sword only. Digging Duo is repeatable.
Fish- One can be found in Dusty Bowl and Stow-On-Side in Shield only. Digging Duo is repeatable.
Next, to go the Revival Machine on Route 6 and present to fossils and they will be revived into a fossil.
For odds, see here (1)https://pokemondb.net/pokebase/350472/which-member-of-the-digging-duo-for-getting-fossils.
Drake + Fish = Dracovish
Drake + Bird = Dracozolt
Fish + Dino = Arctovish
Bird + Dino = Arctozolt