I`m trying to make a really secret base on a friend's realm but i have no idea how to make an item be thrown and a piston revealing the base. Can someone help?
It's somewhat unclear what you're asking.
Like great majority of blocks in the game (rare exceptions being items that manifest as entities: item frames, paintings, armor stands), the pressure plates occupy 1 block space - despite their thin hitbox. And they do work within that 1 block space regardless of whatever block is placed above them - any items/entities placed on them (within limitations; stone plates - only mobs/players; weighted - set number of items) will produce a signal. They do not detect blocks - a block, even a gravity block like sand, placed above them will not be detected (though sand falling from above will break into the item form and the item will be detected).
And obviously they will not detect items on blocks above them - only items on the plate. So if you place the plate under ground, the item on top of the ground won't influence it.
Consider instead using a hopper - it can pull items in through blocks shorter than a full block - e.g. farmland, path block, bottom slab. Reading the hopper content with a comparator will produce a signal, which you then can e.g. send to a T flip-flop circuit to toggle the door (and then retrieve the item from the hopper through an underground passage from within the base).
A hopper minecart can even pull items through a solid block like stone and can also give its items on to a hopper or chest. If it is on a detector rail, a comparator can also detect its contents.