The problem with aluminum (old/bad-style or newer AA-8000) is in the terminations. There are a couple of problems:
- Differing thermal expansion rates. Aluminum terminations can tolerate copper wires because aluminum has a good spring rate. However copper terminations won't "spring" under aluminum wire and will distort themselves or the aluminum, effectively loosening the connection with every cycle.
- Dissimilar metals cause galvanic corrosion. The fix is to use a corrosion inhibiting paste.
Over in Europe, wire nuts and stabs are not legal, and you must use screw-down terminaton blocks. Interestingly, that type is also available in the USA, in the form of the "Alumiconn" splice connector, made specifically to repair aluminum-copper splices. Here's an example of each, see the family resemblance.
(in the Euro connector, the round ports are for the screws, square for wires). And by the way, you notice the Alumiconn is made of aluminum. So are the neutral and ground buses in your panel, also the main lugs. All these applications are also places where you should torque the screws to spec, so they perform properly during dissimilar metal expansion. This is mandatory in the 2017 Electrical Code.
While you have a lot going on in those boxes, they are very deep boxes, not only a double-stacked box with a switch extension. There should be room for Alumiconns, and you'll need something like that on the neutrals and grounds too. Wire nuts involving aluminum Just Don't Work, not even the purple ones supposedly listed for aluminum. Someone might convince me wire nuts can work if all the wires involved are aluminum and the nut is made specifically for all-aluminum, but I don't see anyone developing that product.
One of them is just a straight hot-supply, switched-hot plain switch. Straightforward.
I see a red wire in the mix, so that suggests at least one had once been a 3-way switch. There may be another 3-way switch somewhere else that, if you throw it, the partner black wire to that red becomes energized. (it matters which NM cable the wires are grouped in). Perhaps you have another switch somewhere that "seems to do nothing". Alternately, that black may go back to a failed Al-Cu splice.
Or was there a jumper between the two switches?
There is certainly more going on inside that box than simple switches, but apparently they are all on the same circuit, since the neutrals are all bundled (alternately the person who wired it may be a moron). It may be possible to simplify what's going on in there, perhaps with a little help from smart switches.