I have an old Mac still functional. How can I dispose of it?

see > What to do before you sell, give away, or trade in your Mac - Apple Support

Then use Apple Recycling or drop it off at your local E-Waste Recycler. ♻️

If the Mac is no longer functional, or if it's too old, you can recycle it. Apple's recycling program will take any of your devices and recycle them. They may even give you a gift card if the computer still has some value. You won't get a lot, and lately Apple's prices for Mac trade-ins have been very low

Apple will take care of it for you.

National Services - Apple