Anything (specific) that you create will automatically have
your copyright.
Whenever you create something on your own, you own the interlectual property (ip). You dont need to declare it.

But its good to add somewhere, that others require permission to reuse it.

But a general gameplay idea cant be copyrighted. Only
specific things like art-assets, sourcecode/binaries or
characternames (when they are genue) etc.
But not unspecific things like "athmosphere" or
an "action-puzzle-fashion" game.

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If you have a good idea that you want to keep from
beeing cloned, (and not a finished product),
and you want others to see/work on it,
you can set up a non-disclosure agreement with them.