From this page of the Valheim guide, you will learn how the world is generated in the game.
Every time you start a new game, the game randomly chooses the so-called seed. It determines the shape of the world in which you will play. The whole world is still shaped like a disk with a number of islands scattered across its surface. The shape of each piece of land and the biomes are random. Therefore, when you start a new game, a lot depends on your luck. This is best illustrated by the following graphics showing two different worlds.
As you can see, the two worlds have a completely different layout of islands and biomes on them. The only common point is the biomes located in the north and south of the map. This is because they form distinct stripes and do not appear in any other place. At the time of writing this guide, however, both of these biomes are not completed.