Haven't seen any change on my libido, though mine has always been quite high by default so it could be due to that

I'm not sure how keto could impact testosterone in a negative way, if anything it improves it

I also started working out a while back when I started keto, endurance is a lot easier, cardio I usually have a bit more carbs a couple hours before just to replenish it but it's never that many, and if you do HIIT for cardio you shouldn't really have much of an issue

For muscle building specifically, if your goal is going bodybuilding, I can't really tell what would change. For me I just want stuff toned out so, the improved weight loss and being able to train better and absorb nutrients/protein better has been pretty good for it