[Aevium/Galar-regional form evolutions.]

Aevium Lapras can be found in Amethyst Cave in the Depths. There will be a huge crystal barely hanging at the bottom. You will need to use certain moves that can remove the Crystal Terrain such as Earthquake, Magnitude and even Bulldoze. There should be a Natu wild encounter for you to be able to break Crystal Cave Terrain. The huge crystal should fall after that, interacting it will trigger a fight against Aevium Lapras. You will need Rock Climb to be able to get into this specific place in Amethyst Cave.

[Aevium/Galar-regional form evolutions.]

Xen Waste can be obtained in the den where you originally can get Shadow Beldum's at. While I didn't test it much, I initially filled Beldum's den with 15k Red Essence which you get from doing the Max Raid battles in Route 4. You'll have to go through the process of getting Beldum to obtain Xen waste.