To enter to the Boreal dimension you need to craft the Frozen Amulet, and item that right-clicking in it, you'll be teleported to the Boreal. To craft this item you need one Blue Ice (crafted with 8 compact ices), and 4 Ancient Ice Fragments, which are found in structures in the overworld called Ancient Snow Altars, found in snowy biomes. This structures are made of Boreal Stone, and in the center you will find a chest with one Ancient Ice Fragment. You need to find four of this structures to eb able to visit The Boreal.
Also there's compatibility with Biomes O'Plenty, Terralith and Regions Unexplored, so you should find this structures in snowy biomes of this mods.

In the Boreal when you enter in it for the first time, you will see a boreal wasteland or a beutiful forest, depending on where you spawn. For now there's 3 biomes: Glacial Plains, Glacial Tundra and Auroral Forest. The Glacial Plains is a wasteland of Glacial Grass, and the Glacial Tundra is the same but with snow covering all the biome. You'll find in this biome the Glacial Wolfs, a hostile mob that you can tame with the correct item, and the Frozen Bears, a very dangerous mob that deals a lot of damage and runs a lot faster than you.

Then you have the Auroral Forest, a beutiful forest with tall auroral trees. In this forest you'll find Glacial Wolfs and Night Deers, a passive mob that does strange sounds.

In the Boreal you'll find 3 new ores: the Talismandium, Chrodium and Diorium. These all are find in caves, and you can craft an armor with them, but for the Diorium weapons you also need Diorium Gems, which are find around the Boreal in some of the structures that are in this dimension. You need at least 1 Diorium Gem for every weapon or armor piece of this tier. The Diorium weapons and armor are unbreakable and a lot more powerful than netherite, but very hard to get.