The difference between our previous tutorial and the new one. Part of the content in this tutorial was presented in CIKM 2020, which is the first tutorial to discuss state-of-the-art research works and industrial trends on multi-model data query languages. In the CIKM 2020 tutorial, we mainly focused on the syntax of query languages and their semantic difference. Furthermore, we invited the participants to write and run cross-model queries with UniBench. In this tutorial, we will concentrate on the paradigms for evaluating cross-model queries and strategies for query optimization. We will also provide a hands-on section to demonstrate the cross-model query processing in MMDB systems such as ArangoDB and OrientDB.