Free weight exercises to build upper body? (Pre-T advice wanted but Post-T accepted too)

My fitness knowledge is very minimal. I've been trying to learn but I feel like I'm getting slapped with the combined knowledge of 7 textbooks and 120 research papers at once and retaining none of it.

My primary goal is to build general strength (full body, but mostly upper body) and muscle mass (upper body). My legs are naturally pretty strong and bigger than the rest of me so I'm not too worried about looking disproportional (at least not yet).

I want to get broader shoulders, bigger pecs, and bigger arms. My current problem is I stopped T (it's a long story, I don't want to get into it or hear about all the ways I can get back on it. It's just not feasible right now and it's not the topic of conversation). The weight I can move has decreased dramatically very quickly and I don't understand why. I could do 70lbs reps easy on the double bicep curl machine, but now I can only do reps of 25lbs. Maybe 40lbs on a good day (I usually aim for 8-10 reps a set). I didn't think strength loss would be that quick or that dramatic.

My left side is also suddenly dramatically weaker than my right side, so I can't use machines that require you to use both arms or both legs at once. My right side takes over the exercise entirely. So I have to use free weights or do something else that'll target just one side at a time. But there's also like 101 bicep exercises alone I swear, so how do I know which exercises I need to do to build each muscle? I sometimes see exercises that say they target 1 of 3 parts of the tricep. I thought the tricep was just one muscle? Or I'll see exercises that target the chest but then other people will say it actually only targets upper chest, not lower chest too. So how do I figure out what I need to do?

And what are some tips for building strength and muscle without T? Obviously it's possible but I know it's harder. I don't want to just keep doing random bullshit. That worked on T but I know it's not going to work off T.