Write HTML, CSS and JavaScript and watch the result update as you type, with console.log output underneath. Everything runs in a sandbox in your browser; save the page as a single .html file.
The result runs in a sandboxed frame with no access to this page, your cookies or your storage, so pasting code from a tutorial can’t read anything of yours. Its console messages and errors are shown under the result.
Yes: add a <script src> or <link> from a CDN in the HTML.
On this device, for next time. Share makes a link with the code inside it.
Open all 35 Dev Tools. Everything runs in your browser; nothing you paste is uploaded.