How to best add one part on top of first part

Assumption: I have part A and decided to cut same into two pieces A+B

How can I best add resp. place part B on top of part A matching the exact height. I.e. I want to drop part B on to part A ?

Thanks

I know no other way than to “add part” to A eg. using the STL for B, and then manually enter coordinates for B. It’s tedious though: It’s complicated by the fact the coordinates end up relative, not to mention the fact all coordinates are in the middle of objects as opposed to bottom and some corner. Having Z=0 mean “on bed” would be easier. I would love options such as “drop to top of A” and “drop to bed”.

But I’m a novice so let’s hope for someone to chime in with nicer ways of doing it.