The first question is easy to answer. In Minecraft the distance in the Nether is compacted at a 1:8 ratio compared to the Overworld. What this means is that block 1000,1000 in the Nether is equal to block 8000,8000 in the Overworld. This is why they build paths in the Nether to reach certain biomes as it takes a lot less time to travel in the Nether then in the Overworld before you get an Elytra where it then switches due to how dangerous it is to fly in the Nether.

As to how they know where to build a portal it's due to how Minecraft world generation actually works compared to how a lot of people thinks it works. Most people think it generates terrain randomly in front of/around a player but in reality it doesn't and what's generated in every dimension is actually predestined on world generation through what is known as a "world seed" number. (Which is actually how pretty much all procedurally generated games worlds are generated btw. Minecraft just allows access to the number that's used unlike most games.)

If you know what the said world seed number is (which you can easily find out by just opening up the chat box and typing and entering /seed) you can easily find out what's going to be generated and where using online world seed map databases which have a map of what every seed in Minecraft generates like right down to where ravines and random igloos will generate let alone biomes.

Whether the girls actually use this or it's just management that does and tells them where to go is unknown outside of management using a different command Minecraft has that you can use early on to locate certain biomes in game instead of using a seed mapping database.