The generated style.css defines the font face Untitled.
You can either copy the contents of style.css to your own CSS or link to it in your HTML like so:

<link rel="stylesheet" href="https://i.icomoon.io/public/temp/e9f2fa2124/oneco-ikoner/0/font/style.css?t=1787560592">
<link rel="preload" href="https://i.icomoon.io/public/temp/e9f2fa2124/oneco-ikoner/0/font/fonts/Untitled.woff2?t=1787560592" as="font" type="font/woff2" crossorigin="anonymous" />

The second line is for enabling early font downloads.

Use the CSS class icon to apply the generated font to any text:

<span class="icon"></span>

You could change the size of the glyph using the font-size property in CSS:

.my-size {
    font-size: 2em;
}

<span class="icon my-size"></span>