The files are actually very big. So it would be a lot of work to do it manually. Do you have any suggestions for me?
Snap Geometries to Layer tool, which you can find in the Processing Toolbox.
Snap geometries to layer
This algorithm snaps the geometries in a layer. Snapping can be done either to the geometries from another layer, or to geometries within the same layer. A tolerance is specified in layer units to control how close vertices need to be to the reference layer geometries before they are snapped. Snapping occurs to both nodes and edges. Depending on the snapping behavior, either nodes or edges will be preferred. Vertices will be inserted or removed as required to make the geometries match the reference geometries.