I am currently designing a custom PCB and would like to have an integrated ST Link just as in the discovery modules, where could I find the ST Link firmware for this? Thanks a lot in advance.

[TDK]

It's not publicly distributed.

The STLINK-V3MODS is designed to be soldered to your board for this application. You could consider that.

https://www.st.com/en/development-tools/stlink-v3mods.html

[THooi.1]

Thanks for you quick reply, I was actually thinking this would be the case. Thanks for the verification!