SymbiYosys (sby) is a front-end driver program for Yosys-based formal hardware verification flows. See https://yosyshq.readthedocs.io/projects/sby/ for documentation on how to use SymbiYosys.
SymbiYosys (sby) itself is licensed under the ISC license, note that the solvers and other components used by SymbiYosys come with their own license terms. There is some more details in the "Selecting the right engine" section of the documentation.
SymbiYosys (sby) is part of the Tabby CAD Suite and the OSS CAD Suite! The easiest way to use sby is to install the binary software suite, which contains all required dependencies, including all supported solvers.
Make sure to get a Tabby CAD Suite Evaluation License for extensive SystemVerilog Assertion (SVA) support, as well as industry-grade SystemVerilog and VHDL parsers!
For more information about the difference between Tabby CAD Suite and the OSS CAD Suite, please visit https://www.yosyshq.com/tabby-cad-datasheet.