I think you can generate your note using markdown format (notion, apple notes etc support it) and then you can use the share api to open the file using notes

Using Navigator: clipboard

It is possible to save html to the clipboard.

1. Format the notes in a structured html
2. Create a button with a click event
3. In this click event use the structured html and navigator.clipboard.write to save the html into the users clipboard
4. The User will now be able to paste the content into their notes and they will be formatted perfectly.

Small Code Example

    // first create the clipboard item - use .then to fetch the data because otherwise it will not work on ios
    const clipboardItem = new ClipboardItem({
        'text/html': getYourContent().then((htmlContent) => {
            return new Blob([htmlContent], { type: 'text/html' });
        }),
    });

    navigator.clipboard.write([clipboardItem]);


    const getYourContent = () => {
        // fetch content if you really use this
        // small html example:
        const htmlContent = `
              <h1>Header</h1>
              <p>$paragraph</p>
              <br/>
              <img src="https:/picture.com" />
        `;
        
        return htmlContent
    }

On macOS, you could create RTFD bundles.

Rich Text Format Directory (RTFD) bundles, such as those created by TextEdit rich text documents with images, can be imported into Apple Notes on macOS with formatting and images preserved. The bundle is essentially a directory with a standard Rich Text Format (.rtf) file, along with attachments (such as images).

If you can package and allow downloading content from your application as RTFD bundles, they can be easily imported by the user, by just opening them in Apple Notes. I would suggest creating a rich text document in TextEdit (on macOS) and adding images, then opening the resulting RTFD bundle in Apple Notes. If this solution works for you, you can inspect the underlying file structure of the bundle to see how you can make an export of your application data that matches the RTFD bundle structure.