Hi Lang, and it sounds like you've got it all worked out now?

Just to clarify - you got your original behavior because your object was not a closed solid, it had openings at the top and bottom.

In order to get the booleans to behave as you were expecting originally you need to close off those openings in order to make your object to be a solid volume.

Booleans are mostly oriented towards working with solids, with it deciding which pieces to keep and which to discard based on which volume they are contained inside of so usually if you plan to use booleans you want to form solids out of your pieces before using them. If you don't want to form solids then usually you will use the Edit > Trim command to cut surfaces up rather than booleans.

Booleans are kind of like a sort of "batch mode" that does a similar job of Trim + deleting things depending on which volume they are inside of + joining. You can do the same thing by manual steps of Trim + delete + join but if it is possible to work with solids the booleans tend to be more convenient and can save some steps.

- Michael