I don't have experience with this, but from talking to people who do, there are indeed open source implementations. They say the documentation is a bit confusing, but it does describe everything.

This Rust library is an abstract USB library that can implement CDC serial https://docs.rs/usb-device/0.2.5/usb_device/, and this library connects it to the STM32 USB peripheral https://github.com/stm32-rs/stm32-usbd

Examples here: https://github.com/stm32-rs/stm32-usbd-examples