Icons are often needed for a menu toggle or search bar.
A basic icon could be implemented as a background image on a div or pseudo element.
An svg can scale for a higher quality icon at a smaller size.
SVG or icon sprites are a single file with your site's icons. They are used to save bandwidth when you have many different icons.
In practice, a designer selects an icon to use in a mockup. A developer then has to find a way to get it into the application. This has traditionally been an obstacle that took time to arrange the right file size and format. There are icon sets and tools available that can streamline the process if both sides agree to use the same provider.
Many of these tools are freely available. Check out https://iconify.design/.
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