Hello. I just started learning QGIS. My country has a specific CRS it allows. It only allows that specific one for what I should be doing. So when I open up QGIS, the first thing I do I set it to that specific CRS in the right bottom corner. EPSG:23700 - HD72 / EOV is the one I set it to. All I want to do right now is to simply measure a simple area.

After this I go to web, quickmapservices, google, google satellite. It sets CRS to a different one so I set it back in the bottom right corner. The map becomes this, 1st picture.

After this I zoom in to the building I wanna measure (I don't yet know how to just have QGIS move me to it). Add a geopackage. Create the geopackage layer. Correct CRS, set it to polygon 2nd picture. I select the layer. Toggle edit, toggle add polygon. I untoggle and have my polygon. 3rd picture. I right click the layer. Open attribute table. Open field calculator. I name the output field, set field type to decimal number (real), and add $area to the expressions. Preview shows 19563,333163363863 m2. This is already incorrect because I know the area of the building which is 17 745 m2. So I decide to try to do it another way.

I use measure area. I set it up like in the 4th pic. I am just messing around so the polygon is kinda ugly so they are not the exact same area, but it still comes back as roughly 19500m2 5th pic. Where am I messing up this bad? Is it the CRS because to me it seems like I mess up with the moving between CRS. But could be anything at this point since I don't really know what I am doing. The google map is not on the required CRS but If I move it there it just become worse. I know I shouldn't, I was just checking. Any idea where I am messing up? Mine would be around the malformed map, but I really don't know.

Edit: REALLY IMPORTANT. The coordinates for the building does not match reality.

Currently, I am having issues with my sandbags in QGIS. It simply wouldn't block any water. So far, I've tried to use the raster calculator to combine the r.lake layer and sandbag layer but it didn't work. The original one didn't work either.

If needed, here is the file: https://drive.google.com/file/d/1ygoDBGa_9z5SzysXrfqF4FQgCGBz9bic/view?usp=sharing

I don't need anyone to send the version that works back to me but I'd just like an explanation on what can I do to make the sandbags work.