Source code for https://victorfont.com/wordpress-database-modernization-blueprint/

wpdb-determine_charset.php

<?php

public function determine_charset( $charset, $collate ) {
    if ( ( $this->use_mysqli && ! ( $this->dbh instanceof mysqli ) ) || empty( $this->dbh ) ) {
        return compact( 'charset', 'collate' );
    }

    if ( 'utf8' === $charset && $this->has_cap( 'utf8mb4' ) ) {
        $charset = 'utf8mb4';
    }

    if ( 'utf8mb4' === $charset && ! $this->has_cap( 'utf8mb4' ) ) {
        $charset = 'utf8';
        $collate = str_replace( 'utf8mb4_', 'utf8_', $collate );
    }

    if ( 'utf8mb4' === $charset ) {
        // _general_ is outdated, so we can upgrade it to _unicode_, instead.
        if ( ! $collate || 'utf8_general_ci' === $collate ) {
            $collate = 'utf8mb4_unicode_ci';
        } else {
            $collate = str_replace( 'utf8_', 'utf8mb4_', $collate );
        }
    }

    // _unicode_520_ is a better collation, we should use that when it's available.
    if ( $this->has_cap( 'utf8mb4_520' ) && 'utf8mb4_unicode_ci' === $collate ) {
        $collate = 'utf8mb4_unicode_520_ci';
    }

    return compact( 'charset', 'collate' );
}