How do 2nd moves work in gym defence?

As I understand, it's partly random.

A Pokemon has a 50% chance of using one of it's charge moves, assuming it has gained sufficient energy. So if a defender both has a high energy move (e.g. a one-bar move like Solar Beam) and a low energy move (e.g. a three-bar move like Seed Bomb), then the latter should theoretically get used more often. With two equal energy charge moves, it should be entirely random.

It would add a tiny but appreciated layer of depth to an otherwise awful gym system if the defender automatically used the most effective charge move

I'm pretty sure they choose based on opponents' weaknesses.
By the way, your English is good!