Regarding Joining (Merging) Layers

I have searched everywhere, trying to figure out how to move items to the main layer without deleting the main layer contents. I have just joined the forum, so this is my first post here. Basically I have my main layer for a house I am working on, and on the second layer I am doing plants and such, because the first layer is quite laggy.


You move selected objects from one layer to another with shortcut M. Holding the shift key allows you to assign them to multiple layers