How do I dispose of my 15" old MBP?

Do the following: What to do before you sell, give away, or trade in your Mac - Apple Support

Then, if it has no value and you want to recycle it, options include doing an internet search for electronics recycling in your area, or taking it to an Apple Store or use the following to recycle it online https://www.apple.com/shop/trade-in -- note that in the get an estimate section it will either offer you a gift card or offer to recycle it if it has no value. If you have a Best Buy near you, they also offer recycling.