Just to make it even more real: During covid I added a sub-panel and the wire (more like the sausage given the girth) between the sub-panel and main panel was aluminum because of cost. You just need to be a tad careful at the connection points with copper -- nothing a caring literate person can't handle

The incoming drop for a service panel is almost always aluminum (at least the residential stuff I've seen). It's cheaper and lighter than copper while still being ductile enough to work with. Just use the proper antioxidation paste at the connection point.

Did you use antioxidant paste on all of the aluminum terminations? If you didn’t, please pay an electrician to come fix it. If you did apply the paste, you are in the 0.01% of non-idiot non-electrician homeowners and can skip my rant about homeowners doing electrical work in the next paragraph. I sell electrical work and have seen some incredibly poor work done by homeowners.

Do not install aluminum wire if you are a homeowner unless you already know enough to use antioxidant paste and also to use a torque screwdriver or torque wrench for terminations and know where to find the torque values for the wiring devices you are using. If you don’t apply the paste, the surface of the aluminum will oxidize and could catch fire due to increased resistance.

I suggest not even touching copper as a homeowner, but it’s your choice and your house.