A CI quick tip to help us enable Auto Intersection is posted below.
You can also fix the 90 degree corner join by selecting both walls and clicking the corner join icon. Please see this excellent video to see how by an expert Archicad user.
Like Ling says above, the walls need to be grouped together but before you do that you usually need to draw walls using the continuous method. (see the icon in the walls drawing methods at the top of your screen).
Like DGS says, your base lines need to meet but if you draw with the method I explained above it should do that automatically.
Archicad gives you more control at how walls behave so you have to know how it works first.
These tips & tricks from this excellent video from Shoegnome Architects below may help you out too ? Jared Banks is one of the Archimasters of residential Architecture.
AC7 EDU & AC8.1 - AC26 + Ci Tools
Apple Mac Studio M1 Max Chip CPU 10C
GPU 24C 7.8TF 32GB RAM OS Monterey