Canon 5D Mark IV 'problem' or my 'ignorance'

After having changed to mirrorless (owning now the Canon R6 Mark II and Canon R7), last week I decided and bought the fullframe Canon 5D Mark IV (I own the APS-C Canon 7D). Out of the box, battery and memory card ready, I turned upset/nervous with something: Only in Av (or also in Tv mode), choosing an aperture (i.e. f/8), no matter the different brightness I pointed the camera to, the shutter speed (or the aperture in Tv) remained always the same, turning the file underexposed or...