When you enable this datapack (before you start the world!), you will spawn in the nether. By default, you will respawn in the nether.

Technically, you spawn in the overworld, but the only apparent difference is seeing minecraft:overworld in f3. Everything else in the "overworld" is exactly like the nether, and everything in the "nether" is exactly like the overworld.

Everything works as expected: beds, respawn anchors, piglins, coordinates.

The seed will always be the same because of this bug: https://bugs.mojang.com/browse/MC-19571 (so please go vote for this issue).
If you want to use a different seed, go into the overworld.json and the_nether.json and change both the seed values in each file (you should be changing 4 seed values total). Make sure you extract the folder before editing the values.