Email Signature Generator builds your signature using the table-based HTML that email clients actually render correctly, not the modern CSS a web page would use, so it looks the same when you paste it into Gmail or Outlook as it does in the preview.
Email clients, especially Outlook, use far more limited and inconsistent HTML/CSS rendering than a web browser: no flexbox or grid, and often only inline styles. A table-based layout with inline styles is the format that actually renders correctly and consistently across Gmail, Outlook, and Apple Mail.
An uploaded photo is embedded directly in the signature as a data URI. Some email clients, Outlook in particular, can be unreliable about displaying embedded images this way. If that happens, host the image somewhere (like your company website) and use the "Image URL" option instead, which is the more broadly reliable approach.
Email clients strip out <link> tags and external @font-face/@import CSS, the mechanisms a custom web font needs to load, so it would just fall back to a default font anyway. The font list here is limited to "web-safe" fonts (Arial, Georgia, Verdana, and similar) that are pre-installed on nearly every device, so the signature actually renders in the font you picked.
It copies a simple text-only version of your signature, useful for plain-text email settings or anywhere rich formatting isn't supported.
No. The signature is generated and copied entirely in your browser. Nothing you type is sent to a server.