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FS25 Terrain Editing Overview
Older farmsim game tutorials definitely help beginners, do not make the mistake of thinking that "I don't need to read old FS19 tutorials!!1" when most of the fundamental stuff still works the same, some smaller nuances have of course changed but the general knowledge is still the same.
If you are beginner first read Terrain Editing Tutorial FS17, Terrain Editing Tutorial FS19 and Terrain Editing Tutorial FS22. There is a lot to read so don't be alarmed, take it nice and easy, you'll get there eventually 
Giants Editor v10 page links to multiple other pages, there is a lot to read about Giants Editor v10.0.x usage.
Foliage RGB Colors for painting grass, flowers, shrubbery, bushes, etc in GIMP.
Giants Broke Large Terrains find out what happened to large terrains with FS25.