Linedef executors are SRB2's hacky way of implementing many effects (like teleports).
You should read this article about them on the wiki, but when actually using them, you just need to refer to each one's special info.
If you think of each field -- that is, each value -- as just a setting for the effect, it becomes a bit less confusing. If you have the SRB2 config for Zone Builder, you don't have to worry about details like what number of flag makes the teleport silent, since the flag names are replaced with toggles for options the linedef executor has. I always check the linedef special info section first, then I look through the details to find info about settings that aren't immediately obvious. Once that's set up, and your effect trigger sector and activation linedef are set up, you're good to go.
If you're having trouble, the best way to learn about how each linedef executor works is to open up a relatively simple map that uses the effect you want to set up and give it a good look. You can find stuff using the search function (F3) and check out how they connect to control sectors and other parts of the map. I think almost every linedef executor has an example level you can download.
As for making bosses, custom ones are usually done using Lua these days. That's a bit more involved, so don't feel bad about reusing ones that come with the game! Just don't make exact copies of our boss arenas :p
If all else fails, you might find people to collaborate with by sharing screenshots of what you're doing in the Works In Progress forum.
I hope this help you on your modding journey. Good luck.