Does bicep development affect punching power?
Yes, it does. I can't give you an explanation based on physics or biology; I will try to explain this in simple terms.
Here is how bicep development can affect punching power
Where do you punch from? Are your hands inside your jacket pockets before you take them out and punch? No. Your hands are up high! They are around your face/neck, guarding your chin. Which muscle groups help keep your hands glued to your face? Your biceps and front delts (i.e., the anterior deltoids) are the main set of muscles that help you keep your hands up. This is important because when you keep your hands up, you are essentially always using your biceps (i.e., through contraction). And don't forget that your biceps are constantly holding up some additional weight (e.g., ~ 17 ounce = wraps + 16 ounce bag gloves).
As we go through the rounds, our biceps get tired from the constant contraction; that is when our hands come down from our chins (and the coach yells "Oye, keep those hands up!"). This is also when we can't throw strong punches - the cross is strongest when it travels in a straight line. If you can't keep your right hand at chin-level, your cross won't travel in a straight line.
Here is an example: Let's say you could throw a maximum of 5 rear-straight punches before the Covid19-training. Your fifth punch was the weakest of the five because by this time, your hands were fully tired.
During your Covid19-training, you did some from of weight training that increased your strength. Both Bicep Curls and Push-ups strengthen your biceps. Now after the Covid19-training, you can throw a maximum of 8 rear-straight punches. The eight punch is your weakest now.
So it is quite possible that your post-Covid19-training fifth punch is stronger than your pre-Covid19-training fifth punch. This is because your biceps are now stronger than before, and they can keep your hands up high for much longer. So after the Covid19-training, at your fifth punch, your biceps are not fully tired.
Think about it this way. A beginner cannot bicep curl 40 lbs (i.e., a single dumbbell) on the first day. He is probably going to start with 25 lbs. But after 6 months, he will be able to curl dumbbells weighing 40 lbs. At this time, 25-pound dumbbells will feel much lighter and easier to curl. WHY? Because his biceps have grown stronger, they can easily handle 25 lbs. Similarly, your biceps have grown stronger. They can now easily keep your hands up high along with the weight of the gloves.
"Stronger muscles last longer. It takes longer before they get tired because every movement takes less effort than before." (Stronglifts - see Endurance)
Other factors that improved your punching power
... most of what I did for exercising was running, shadowboxing and some weightlifting (squats, push-ups and bicep curls)
First, let's look at the The Anatomy of a Punch:
The force used to deliver a powerful punch begins from the feet – when we push into the ground, the ground pushes back into us, followed by a sequence of muscular contractions that transfers force up our kinetic chain until our fist hits our target. The transfer of force happens quickly through a series of rotations and torques from the foot, up the legs and thighs, through the trunk and core, and finally out through the shoulders and arms.
The muscles of the shoulder are key players that deliver the punch, but also for taking the brunt of the impact. The anterior deltoid along with the pectoralis major, the muscles of your chest and shoulder, project your arm out towards your target.
The internal and external obliques are responsible for transferring rotational force without any energy leaks by having the rib cage and pelvis turn as a unit.
The quads and hamstrings help transfer force from the knee to the hips.
There are many factors that make a punch stronger. When you throw the right cross, you engage several things - your core, your hips (the turn), your lats, your biceps and triceps, your shoulders, and your chest.
Push-ups strengthen your triceps, your shoulders, and your chest muscles (i.e., the pectorals). Together, they all made your punches stronger.
Running may have helped with over all endurance. And squats do work your core in addition to your legs, both of which are massive contributors to effective punching. Even bicep curls, when done standing up, engage the core. Your squats made your legs stronger, which means you can go for more powerful bursts of flurries now (power transfers through the kinetic chain), and more often than before.
Add your shadowboxing to these, and you have a good explanation for why your punches are stronger now. Your core, shoulders & chest (which are mainly responsible for shooting/projecting out your hands), biceps (partly responsible for keeping your hands up and cocking your hands back), and your triceps (mainly responsible for extension or straightening of the arms) have all become stronger. Your push-ups clearly worked your triceps. This means more faster and snappy punches.