Me and my brother recently started a survival world online with eachother on separate consoles. We explored a bit then came off but when we came back on there was a wooden cross with two chests each with 4 stack of 64 diamond blocks and 64, gold blocks. A few days later we found a wooden structure with a massive white stack of unknown blocks floating above a wooden structure. Upon mining the blocks it gave us nothing back. Then about 20 minutes later we found an emerald block pyramid with two beacons on top. Just to say none of these things were here when we spawned in. We have also found it very easy to find valuable ores such as iron and gold.
I do not think they were placed there by my brother as we always play together and he was genuinely shocked upon finding them. Our world is also private and we're are the only ones who have been in it.