well, over the years I've had several builds i wanted up upload, but part of minecraft is making something unique to you, not everyone has the time or patience, and sometimes even the ability to make a huge build come together without it being a giant mess, and that's OK too because you can always rebuild, We've all started out with dirt huts or had one at some point =)

as far as plagiarism: knowing how many people play minecraft and the majority of players play with default textures, at one point or another there are bound to be builds that look near similar, though like myself i prefer knowing my server is 100% block-by-block with creative mode on server areas just because i like knowing It was built from the grass blocks up. Again, players just starting servers and especially those totally new to it of course want the assembly of spawn and world locations added as quick as possible, and just pasting a giant structure or even a simply unique one is the faster option for "their" way of doing things sometimes, to each their own.

What to share: I usually only post things i know are not Too specific to the point most people won't find a use for it, or things like Server areas and structures I'll typically keep hush, but arena's or other things that have a straight-forward application, I'm more than happy to share it. but things you'll see when you first join like a HUB area or buildings you have for a specific purpose, I'd keep those gems to myself knowing i built them myself, though of course they can visit the structures anytime, but on "My" server lol =)

Why i share: I uploaded the first arena i built on my server just because it came out really nice, Plus i was excited and wanted to share it among the community here because after it was complete, i figured it's just the right size if some one just wants a 40x80 arena to do as i did years ago, and start face rolling the keyboard as you learn the ropes of setting up arenas on a server if they've never seen a config file before, a 1v1 or 2v2 would be perfect for them to learn on >.<
I'm planning to make much more detailed and larger arenas, so I'm not too worried if i see my creation i shared willingly on another server, I take it as a compliment every time the upload counter goes up!

Down Right Copy Cats: Making a house or civilian structure is one thing, but making an exact duplicate of another server's spawn builds to claim for their own spawn is pretty low. especially if they go as far as to troll the server they stole it from.
I'm fine with my builds giving inspiration if they ever do, as long as they credit the original maker in some way even if it's just a sign on it, or at least just simply ask first. but if the person is trying to claim the design as their own when they knowingly copied it, down to the items in item frames and flower colors or literally every block on a 100x100+ build, I'd hope for Karma's sake they credit who inspired them. kinda scummy to take little timmy's crutches then turn and hit him with it; It's not hard to be civil minded in MineCraft.

Stopping the Copying: I think it would be epic if there was a way to "encrypt" a schematic file so it cannot be changed, kind of like a PDF file would work, they are able to do it with plugins, but i honestly know nothing about coding yet or how to make something like that work... LOL

so far every time i saw a stolen skin of mine i reported it and it got taken down pretty much immediately. i don't think you need to worry about it too much because you can always prove it is yours since minecraft sites and social media always have a date on when a build was submitted

Before I even begin to dive into the deep end, I'd like to start off by saying that this is my personal opinion. PMC isn't exactly the best place to share MC builds. I've found the staff on this site to be unreliable, abusive, and biased toward higher level members. I had made a Snowtrooper skin from Star Wars on one of my previous accounts that was made from the ground up on Skindex. A few days/weeks later, a higher level member made a Death Trooper skin that had the exact same pixel pattern as my skin's visor on the helmet. When I reported for the design clearly being stolen, the staff did nothing about it. That being said, reporting plagiarism to the staff probably won't do much, but it's worth a shot anyway. If someone is literally stealing something entirely (as you mentioned, a screenshot or map file that you clearly made first) then there's a chance staff will do something about it. Not a very good chance in my opinion, but it's still better than nothing.