Help!!!! Regarding isofix base for car seat

Not 100% sure on this isofix stuff for baby's car seat. Can the car seat still be used ok with or without the isofix base? Like is the base only really for more security without the seatbelt being on? Or is it a must now that all car seats MUST have an isofix base for the seat to clip into? So confused would appreciate any help given, thanks!😫🙈