If you just want to play with other people and you don't really care who, servers are the better option because there are tons of them. There is a whole section in PMC dedicated to servers. If you only want to play with people you know then you might go witj realms because no one can randomly join.
I've never personally played realms but I know people who have and from what I understand it's essentially a server that is ran by Minecraft. Unlike servers you have to be invited to one or make one and invite others. Hope this helps
My friends and I have seen the same thing several times as we needed a lot of beacons so we were in the nether for a while. It happened at least twice and we were around 1000 blocks from the nearest portal
Java is more customizable but it can be harder to play with friends. Bedrock doesn't have great redstone but multiplayer is better for playing with friends