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 overexposed! never that had happened before with any Canon camera I had owned. So, or there's a real problem with the upper left dial only in Av or Tv modes (not in P or M), or... for the first time I feel dumb with the menu of a Canon camera! (No, no exposure limits reached, nor ISO problem: tried setting to 100 and also in auto). Something else to try? Please help! Thanks, guys!