X / Twitter QR editor.

Use x.com/yourhandle. The old twitter.com URLs still redirect, so either domain works, but x.com is current.

On phones with the X app installed, x.com links typically deep-link into the app. Without the app, the link opens in a browser.

Public profiles and posts are readable in a browser without logging in.

No. Public X profiles and posts are visible to anyone without an account. Some content (replies, certain threads) may be partially hidden until the user logs in, but the main profile and timeline are public.

Yes. Open the post, click Share, and copy the link. It will look like x.com/yourhandle/status/1234567890. Paste that directly into the editor.