I would like to know if there is a way to select the part of a layer that is located at the left (or at the west) of another layer. I thought it would work with select by location, but I don't know how to do it.
You can do the requested task using not Select by location but Select by expression.
In the image below you can see one example of the necessary expression that selects all the features that have an X coordinate smaller than the
X_min (minimum X coordinate) of the polygon.
This selects all the features on the left side. This is the expression:
--features on the left $x < x_min(geometry(get_feature('polygon','id',1) ))
If you want to select all the feature on the right side just change the symbol
NB: This expression work with points. If you want to select lines or polygons, using the same expression, the X coordinate of the selectable layer will be the centroid of the geometry.
To change from the X centroid to the extreme coordinate, you must change the
x_min($geometry) according to the side you want to select.
Example with lines:
x_max($geometry) < x_min(geometry(get_feature('polygon','id',1) ))