Strings from crypto.getRandomValues — the generator your browser trusts for TLS — with rejection sampling so no character is favoured. Pick the alphabet and length, read the honest entropy line, copy. Generated on your device, stored nowhere.
~191 bits of entropy per string — beyond brute force
Set what you want and the result appears as you go. Nothing needs uploading because there is nothing to upload — you are describing what to make, not sending us anything.
Your browser does all the work locally, using your own operating system where randomness is involved. Nothing is requested from a server, so nobody else has a copy of what you made.
One press puts it on your clipboard, ready to paste where you need it. There is no file to download — the result is text, not a document. Nothing is kept — refresh the page and it’s gone, so take what you need before you leave.