One popular server implementation is Vaultwarden (originally called bitwarden_rs), implemented in Rust. [1] The Vaultwarden wiki [2] has more details.

[1]: https://github.com/dani-garcia/vaultwarden

[2]: https://github.com/dani-garcia/vaultwarden/wiki

The popular alternative is bitwarden-rs, and it's usually chosen because it has a much smaller footprint and is well suited to self-hosting where you only have a handful of users.

The official backend is multi-container and uses a full SQL server install.

Minor nit: they've had to change the name to "vaultwarden".

https://github.com/dani-garcia/vaultwarden#readme

> Note: This project was known as Bitwarden_RS and has been renamed to separate itself from the official Bitwarden server in the hopes of avoiding confusion and trademark/branding issues. Please see #1642 for more explanation.