All of your 'raw' geometry [edges, faces, curves, arcs, circles, polygons, surfaces] should always be made on Layer0 - do not change the 'current layer' away from that without understanding the ramifications...
You then select the geometry and put it into a 'container' - either a 'Group' or a 'Component' [these are best for repeated elements, where editing one edits all instances].
You then assign any desired Layer to that 'container' - creating the Layer[s] as desired.
Layers are used to control visibility - they do NOT separate or lock geometry - that is done by using 'containers'.
In you example...
With the current-layer set to Layer0 in the Layer dialog.
Draw the floor slab face on Layer0, extrude it using PushPull etc... and then select all of the new geometry and make a Group.
Select that Group and use Entity Info to reassign it to an new Layer - e.g. 'FLOR-1' [which you make in the Layer dialog]; also take the opportunity to name the Group logically etc - e.g. 'Floor#1'
The Floor-Group is now safely separated from the rest of the Model and by switching off its Layer you can make it invisible.
To change the Floor's geometry Edit the Group and you are then working within the 'container'. Any changes are safely separated - you can inference and snap-to - but not 'stick-to' - external entities...
To create the Walls' geometry I would Edit [go into] the Floor-group and double click its top face to select it and all of its edges, then re-click on that face holding Shift+Ctrl to remove it from the selection.
Edit > Copy.
Exit the Edit and Edit > Paste-In-Place
The perimeter is now replicated in the Model.
While it is still highlighted make that into a Group, named say 'Walls#1' and assign to Layer 'WALL-1'.
Edit that Group and continue making the Walls' geometry inside it, on Layer0.
You can still infer/snap-to objects outside of the context or within it.
Selections are limited to the context's entities.
If external object get in your way, you can 'hide' them by temporarily switching their assigned Layers off.
Alternatively toggling View > Component Edit > Hide Rest of Model... ON/OFF will hide/show everything outside of the current Edit context - allowing you a totally uncluttered view when appropriate, although of course you can't then see external objects to infer/snap-to etc...
