Modern favicons need an SVG version, a PNG fallback for older browsers, and an Apple Touch Icon for iOS home screens. Link them in the head element. Include a manifest.json for PWA and Android home screen support.
Favicons
<!-- Basic favicon -->
<link rel="icon" href="/favicon.ico">
<!-- Modern SVG favicon -->
<link rel="icon" href="/icon.svg" type="image/svg+xml">
<!-- Apple Touch Icon -->
<link rel="apple-touch-icon" href="/apple-icon.png">
<!-- PWA manifest -->
<link rel="manifest" href="/manifest.json">