Register a Self-Hosted Font in theme.json

// Font settings. Files live in wp-content/uploads/fonts/.
$wpide_font = array(
    'name'     => 'Inter',
    'slug'     => 'inter',
    'fallback' => 'system-ui, -apple-system, sans-serif',
    'files'    => array(
        '400' => 'inter-400.woff2',
        '700' => 'inter-700.woff2',
    ),
);

add_filter( 'wp_theme_json_data_theme', function ( $theme_json ) use ( $wpide_font ) {
    $uploads = wp_get_upload_dir();
    $base    = trailingslashit( $uploads['baseurl'] ) . 'fonts/';
    $faces   = array();

    foreach ( $wpide_font['files'] as $weight => $file ) {
        if ( ! file_exists( trailingslashit( $uploads['basedir'] ) . 'fonts/' . $file ) ) {
            continue;
        }

        $faces[] = array(
            'fontFamily'  => $wpide_font['name'],
            'fontWeight'  => (string) $weight,
            'fontStyle'   => 'normal',
            'fontDisplay' => 'swap',
            'src'         => array( $base . $file ),
        );
    }

    if ( ! $faces ) {
        return $theme_json;
    }

    $data     = $theme_json->get_data();
    $families = isset( $data['settings']['typography']['fontFamilies'] ) ? (array) $data['settings']['typography']['fontFamilies'] : array();
    $families = isset( $families['theme'] ) ? (array) $families['theme'] : $families;
    $families = array_values( array_filter( $families, function ( $family ) use ( $wpide_font ) {
        return ! isset( $family['slug'] ) || $family['slug'] !== $wpide_font['slug'];
    } ) );

    $families[] = array(
        'name'       => $wpide_font['name'],
        'slug'       => $wpide_font['slug'],
        'fontFamily' => '"' . $wpide_font['name'] . '", ' . $wpide_font['fallback'],
        'fontFace'   => $faces,
    );

    return $theme_json->update_with( array(
        'version'  => WP_Theme_JSON::LATEST_SCHEMA,
        'settings' => array( 'typography' => array( 'fontFamilies' => $families ) ),
    ) );
} );