Newby question.Thanks

how do I get walls to not merge with slab?

Use groups and components to separate geometry.
Create your slab, select it, right click and choose make group. Then the walls and so on.
To edit a group simply double click it to open for editing, then click outside the group to close it when finished.
A very basic example
GIF 19-12-2021 6-28-46 PM

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