Behind what specifically?
Environment variables have been a feature of just about every OS since the 1960s, They're data items that have a label and a value that executing programs can access at run time. See ==>>
https://en.wikipedia.org/wiki/Environment_variable
Windows Explorer->Send To first surfaced in Windows 95 in the mid 1990s. It is implemented via a folder of shortcuts (destination folders or programs) that is stored in a specific location, usually C:\Users\
<xxxxxx>\AppData\Roaming\Microsoft\Windows\SendTo. It's similar to the 'Desktop', but it is accessed via File Explorer - so data can be sent to a program rather than fetched by program.
BR