First things first, I am completely new to this so please bare with me if this is a bit unclear as I lack the vocabulary.
I have two raster layers (a DEM and a DSM). The DEM layer covers a large rectangular region while the DSM only covers a part of the same region (the Urban part) and the shape of the area is a complicated polygon.
I want to produce a single raster which will have all the information from both DEM and DSM. When I try to merge those two, I get a valid result (everything aligns nicely) however, it looks like the DSM is being projected on the DEM and this results in gaps.
I have high hopes that the attached image will make the situation clear. Below you can see the result of merging the DEM and DSM rasters in 3D View (top) and 2D view (bottom).
PS: I tried to use GRASS
r.patch but unfortunately its UI doesn't allow to chose the order of the layers in QGIS 3.10.