To get the list of databases, schemas, objects without loading them to the Metadata Container, use the following code:

using (var sqlContext = new SQLContext())
{
    sqlContext.Assign(queryBuilder1.SQLContext);
 
    // load all databases, not only default
    sqlContext.MetadataContainer.LoadingOptions.LoadDefaultDatabaseOnly = false;
    // load system databases if needed
    sqlContext.MetadataContainer.LoadingOptions.LoadSystemObjects = true;
 
    using (var databasesList = new MetadataList(sqlContext.MetadataContainer))
    {
        // load databases from the root level (sqlContext.MetadataContainer)
        databasesList.Load(MetadataType.Database, false);

        foreach (var db in databasesList) 
        {
        
            using (var schemasList = new MetadataList(db) 
            {
                // load schemas for each database (db)
                schemasList.Load(MetadataType.Schema, false);

                // process schemasList.Items
            }
        }
    }
}

Or, if you don't need a list of databases, but a list of schemas from all databases, you can simplify this sample as follows:

using (var sqlContext = new SQLContext())
{
    sqlContext.Assign(queryBuilder1.SQLContext);
 
    // load all databases, not only default
    sqlContext.MetadataContainer.LoadingOptions.LoadDefaultDatabaseOnly = false;
    // load system objects if needed
    sqlContext.MetadataContainer.LoadingOptions.LoadSystemObjects = true;
 
    using (var schemasList = new MetadataList(sqlContext.MetadataContainer))
    {
        // set the Recursive parameter to true to load items from lower levels
        schemasList.Load(MetadataType.Schema, true); 

        foreach (var schema in schemasList) 
        {
            // process each schema
        }
    }
}

That's it.