I want to make a terrain where the ending point is also the starting point. So, like on the earth you could just go on walking straight and you would reach the point where you started again after some time.
Thanks for your help!
Unity's Terrain system can only create square regions of terrain. So this can't be done as such.
However, you can approximate it, and I'll tell you how I've done it in my project to some success.
Figure out how much terrain you need to cover the "globe", we'll say it takes NxN chunks of terrain we'll call a "tile".
What you do next is you make 9 of those NxN tiles, and arrange them in a 3x3 grid. Put the camera in the center tile of the grid, and whenever the camera leaves that tile, determine where it is on the tile it is on, then change its position to the corresponding position on the center tile.
This will give you a "toroidal" world. I found this was the easiest solution to get the player to see things on the other "corner" of the world map, and then cross into it without graphical issues.
If you have other objects residing on the world, that presents some additional challenges. One thing you can start with is duplicating them 9x and start them at the same relative position of each tile. If they only interact with the player, that should be fine, just whenever the player interacts with 1, the other 8 do whatever that 1 does.
If the other residents of the globe have to interact with each other, you'll need a way to figure out how to make all 9 copies of everything behave consistently, but that's too broad of a question to address here.