Some more examples where I've used slot joints: for the couch (using the middle option), and in the shelves inside cabinet (using the last option):
One thing to keep in mind is that the width of the slot - the width you need to indicate in your CAD drawings - depends on the width of the piece to be slid into it. If you are planning on slotting a piece cut out of 12 mm-thick plywood, then you would think that you would need to draw a slot 12 mm wide.
However, materials may not be exactly the thickness that it purports to be. Plywood is often around 1% or 2% thicker or thinner than the labelled thickness, and this is enough to make a slot joint too loose (and therefore wobbly) or too tight to slide in without a very heavy hammer-like implement.
My first ever design, a fridge stand, had a slot which did not take the actual thickness into account. My slot was exactly 18 mm and the material was just a couple of tenths of a millimetres thicker.
In most cases, I want a slot which is tight but not too tight to need a heavy hammer. The way I have managed this is by asking my CNC machine operator to obtain the material and measure the exact thickness first. I ask them to wait until I have adjusted my CAD file to take the exact thickness into account. Once I send them an updated file, they start milling.
This is now a standard part of every job and it works well. No more heavy hammer work for slot joins anymore.