I have problem to connect this column to the base like in the photo. I would like to connect that parts as one connected edges. Please help :(
We don't know a lot about your topology, but you could try this way: Select your top cube, then click Face > Intersect (Boolean), then change the Operator box setting to Union. You may need to make some adjustments like Remove Doubles (Merge by Distance in 2.8) or draw some edges with knife (or J to join vertices).