what happens to them?.
When I was an elder, one copy goes to the WT Headquarters, one copy is kept by the secretary and filed. All elders have access to the letter and many times if you move to another kingdom Hall, some elders will tell the other elders the specifics of what you did to get disfellowshipped. It's true what Sir82 said, elders don't keep anything confidential. Many swap stories between congregations, others tell the families and friends what the person did, especially sexual acts, and if they get removed as elders or or step down, they are not bound to keep things confidential.They can tell whoever they want.
They will not give you the letter or any copies, those are WT property.