Use custom field values as options in a dropdown or checkbox

Meta value as options

add_filter( 'user_meta_field_config', 'user_meta_field_config_populate_options', 10, 4 );
function user_meta_field_config_populate_options( $field, $fieldID, $formName, $userID ){ 

    if( $fieldID != 'Your_Field_ID' ) // Put your desired field id here
        return $field;
        
    $metaKeys = array( 'key1', 'key2', 'key3' );
    
    $output = null;
    foreach( $metaKeys as $key ):
        $output .= get_user_meta( $userID, $key, true ) . ',';
    endforeach;
    $output = ',' . trim( $output, ',' );
    
    $field['options'] = $output;

    return $field;
}