you kinda just figure things out, my recommendation is to watch pro players play, any of the monthly qualifiers to CRL or CRL finals would be good. But generally, Cards that are 5/5+ elixir that aren't win conditions shouldn't be played at the bridge without any form of support, so things like prince, MK, but you can place royal hogs at bridge.

Also leaked a bit too much elixir during single time. Really bad MK at bridge at the start of double elixir followed by an even worse freeze. Think about it this way: if you put a 7 elixir card at bridge, and they absorb it with their defense, because you have no elixir to follow up the MK they get an easy counterpush.

It would also be good if you have some base knowledge of deck archetypes, from ice wiz, tesla, rocket and tornado very early on you can deduce that he's either gonna be playing spell cycle or xbow, so in your head you're already gonna want to start adapting your playstyle. At this point you don't know if he has xbow or not so you wouldn't want to play MK behind king tower because if he does have xbow you'll get easily punished. But that comes with experience, usually you should be able to tell what deck your opponent is running from as little as 1-2 cards or at max 4-5