Hi @Scalett_Pegany,
If you look through the documentation pages you'll find the Data resources page. Which has a collection of premade classifiers (and the files used to make them) readily available, e.g. SILVA and Greengenes2.
However, if you'd like to construct and curate your own reference database, you can check out our RESCRIPt plugin. We have tutorials for SILVA, GTDB, and RDP, and other approaches. Also, feel free to check out the new Greengenes2.