Without looking anything up, I'd guess "compose-,-c" and "compose-o-a", for the first two, and that turns out to be correct.

I had to try a few things for the degree symbol, but I guessed it on my third try (it's "compose-o-o"). Once I knew that, my first guess for ¿ was "compose-?-?", which also turned out to be correct.

But as the other reply said, you can always look them up if necessary, which you'd almost certainly have to do with the Mac's system of "option + some arbitrarily selected key".

> What about ç or å or other stuff like ° or ¡ and ¿?

Those are all covered:

http://fsymbols.com/keyboard/linux/compose/