It depends on what it is. A lot of these places don't own any of the hardware they just lease it. When the lease is up is when they cycle it out. Then it goes to resellers and often ends up on eBay or bulk sold to lower tier data centers. Depending on what it is maybe even shipped to other countries.

Generally large companies will have an existing vendor that they use to dispose their IT equipment through. They will shred parts like storage devices and anything that can reasonably be resold will be sold through various auction houses.

Sticks of ram will certainly be resold, custom aws motherboards - not so much.

I have seen custom (unpublished) intel cpu parts on ebay before which are almost certainly aws's custom ones.

Almost nothing will get used by consumers - enterprise server gear is designed for heat/air speed/noise/energy cost requirements which are incompatible with consumer requirements. It's recycled only in the sense that a smaller business might be interested in it because at the end of its economic life its now cheap to buy (but not cheap to run).

https://cloudninjas.com/

https://savemyserver.com/

https://unixsurplus.com/