Hello! Many of these questions that I will be answering are questions that are quite hard to find out, hope these help!
What website finds naturally generated structures such as Nether Fortresses, End Cities and Strongholds?
There are many different websites, but chunkbase.com allows you to find not just naturally generated structures, but biomes in the Nether and the Overworld, and your spawnpoint, all it requires is your seed.
How do I find the seed of a realm, java and bedrock (MCPE)
To find a realm seed on java, you will need to ask an administrator of the realm, to say /seed.
To find a realm seed on bedrock (MCPE), you will need to ask THE owner of the realm, not an operator, to go onto the main menu, go onto worlds, check their active realms and click the pencil. From there, ask the owner to go onto download realm, and then wait until its complete. After that, you want to ask him to go into the world which should be the name of the realm and then - Creative/Survival, for example, if I had a survival realm called 'The Realm' I would go onto my worlds and enter the world called The Realm - Survival. After that, ask the owner to go onto the sub-menu and then go onto settings inside the world, then scroll down on 'Game' until you find the seed and boom, you have the seed.