I'm interested how to prune correctly or the best way a citrus tree. I can give an example of an indoor orange tree. Would it be ok to trim the left branch until the bottom leaf you can see in the image and the other branches by one third? I live in the temperate climate. In the winter I've kept the tree in a cool room and in the summer outside.
What is it that you would like to achieve with pruning? Is it esthetic, or to get your tree to grow taller? Or fuller? Does this plant carry any fruit yet, or is it still too young?
Anyways, for esthetic reasons I would prune the left branch completely. To the stem of the plant. If you want the plant to become larger and fuller (in order to develop fruits?), then I would suggest to not prune it yet, but put it in a larger container.
Furthermore, the time of pruning is also important, especially when your plant develops fruits already. If you live in the Northern hemisphere, probably the best time to prune should be early spring (Feb-Apr).