Concrete example:

Your npub:  npub1kuy0wwf0tzzqvgfv8zpw0vaupkds3430jhapwrgfjyn7ecnhpe0qj9kdj8

Your testnet address:  tb1pkuy0wwf0tzzqvgfv8zpw0vaupkds3430jhapwrgfjyn7ecnhpe0qxdufq2

Notice they are almost identical characters.

So I just sent you 1 million testnet sats here:

https://mempool.space/testnet4/tx/e0ac22a16f10e975a04736161c31eb6ad62c06a529b73875da009174a3e0e7e0

I just used a directive in my subkey (see below) to send you the sats.  So much is possible!  

https://primal.net/e/note1mhug930fz8nktztrcurxh2k3vl88x7fl0k05rtmjqxy7cs0vjscqplgruf