What to do with old iPhones?

You can trade the 8Plus to Apple for a gift card. See: Apple Trade In - Apple

You can also have Apple recycle the older devices using the same site.

Or a local electronics recycler or Best Buy in the U.S.

You can also sell it directly to someone (eBay, etc) or to a reputable site such as https://www.macmeanoffer.com/sell-iphone/

If they are in good shape, repair sites like iFixit may pay you for them if they can salvage useable parts from them. Things like displays especially may have value as salvage parts.