Attaching garages to houses with foundations isn’t too difficult to pick up once you understand how it's done. I believe this is the tutorial I used when first learning the technique: https://www.carls-sims-3-guide.com/buildedit/houses/garage.php
There are also YouTube tutorials available if you learn things better that way. Hope that helps and good luck with your new build!
I found a few old pix of some earlier uploads where I first used this method—mostly for attaching garages to homes with foundations— but it's useful for attached greenhouses or rooms with in-ground pools too.