Suppose a road segement starts from house number 100 and ends at 200. When you search a house number 125, the returned location will be at about one quarter of the road segment from the end with number 100. Similarly, 175 will be at about three quarters of the road segement from the same end. This is a very simple linear interpolation.
A waypoint is a user defined POI, like those in Favorites. You cannot add street view to your 3790T. Read carefully again about what I said.
To create a waypoint with coordinates, firstly, use Goolge Map's street view to find the location you want to go to. Sometimes you can see the house number in street view. Secondly, there is a small map at the lower right corner of the street view. Click on the upper left corner of this small map to enlarge it. Thirdly, right click on the person in the enlarged map and select "What's here?". A set of coordinates in degrees should show up in the search box near the top of the web page. Fourthly, on your 3790T go to "Where To? -> Coordinates" and enter the coordinates returned by Google Map. You may need to change the format to degree by clicking the three-bar icon at upper left corner. Lastly, click "View on Map" and click on the three-bar icon to save the location created by coordinates as a Favorite point.
Maps created by reverse engineered MapSource installation will lose address search capability. Add the downloaded gmapsupp.img directly to your 3790T instead (unlock as necessary) and the address search function will be retained.