You mean server? If so, the server is also open source (github.com/bitwarden/server), but the default instance (bitwarden.com) is not totaly free - you have to have a payed subscription for some of the features. If you self-host, then you have all the features (free and/or premium) - and this can also be done with Vaultwarden which is a FOSS alternative to the official server.