For static meshes, textures, materials, and blueprints, you’ll need to first migrate these assets to the target project. Then, copy-paste from source level to target level. That is, in the source level’s world outliner, select all the instances you want copied and press Ctrl+C. In the target level, press Ctrl+V. Migration must be done first because otherwise, you’ll get empty instances.
As far as the landscape goes, I think that most of the info is saved in the level file itself and in not a separate asset. I would try exporting the landscape’s heightmap and the individual texture layers to .png files. You’ll also need to migrate the landscape’s material and the landscape layer info objects (usually under the folder _sharedassets). In the target project, create a new landscape but use the ‘Import from File’ option. Make sure it is the same size/resolution and uses same material as the original landscape. Click Import. Under Paint → Target Layers, for each layer, set the appropriate landscape layer info object. Then right-click and import the layer’s png file.