You want to learn how to use the Intersect Faces option.
You can intersect faces so that the geometry gets created where they touch so that you can then select and delete what you don’t want, or separate things.
Which version of intersect with that you use depends on how the faces interact. A simple way to put it, although not a full explanation, If they are in the same context you can select them and use ‘Intersect with selection’ and if they are separated by being in a group you can go inside the group and ‘Intersect the face with model’.
Here is a quick gif showing intersecting a face with a sphere that is a group.
So if you want the terrain to cut the cylinder and it is a solid group not loose geometry, you need to double click the group to edit then click the face to select it and right click for Intersect with model. This should give you line around the cylinder. You can then select the part you don’t want and delete it.
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