Budvar10:
There used to be STM32duino forum by Roger Clark. He is also member here.
However, it looks like it is not active anymore.
I haven't followed it enough to comment on its value, but there exists an Arduino forum under the broader STM board: https://community.st.com/s/topic/0TO0X000000BWWSWA4/arduino
As noted by "westfw" the backstory for Arduino on STM32 is convoluted and confusing to someone just getting on board and for that matter, those of us who have been at it for awhile. At this point in time and going forward the recommendation is to use the official STM Core.
It's important to be aware that a lot of "how-tos" one finds on the internet were created prior to this point in the Arduino on STM32 evolution and might not be quite right anymore.