Hallo Knowing that several interfaces (USART1, USART3, CAN, USB) can interface to the embedded bootloader I wonder if there is any requriments for levels on thise pins when we want to use a specific interface?

Yes

By pulling BOOT1 pin to HI during power on reset the device boots from system memory in witch resides the embedded bootloader.

AN2606 contains information for all mcu families how to configure and select the embeded bootloader

It is sensitive to signals on all the usable interfaces, make sure the one you plan on using is the only one the loader sees activity on when operating in this mode.