Something non-destructive and pretty easy is using a layer style to make the white background transparent. right click on your layer with the white background and go to “blending options”
in that menu at the bottom there should be an option for “blend if” with two sliders. If you move the white (right) slider towards the center a tiny bit, your background should become completely clear. This doesn’t erase anything, or create a mask, it just makes white clear.
Hope that helps!
BTW easiest way to unlock a “background” layer is to double click on it in the layers panel, and click ok in the new layer menu that pops up. done.