The HSE code is generally licensed under the "Apache 2.0" license with two exceptions. See hse/docs/SourceConventions.md for how this is to be indicated within the HSE source files themselves.
The hse/3rdparty directory contains code not written by the HSE team but used by HSE. Such code continues to be licensed under its original terms, but may exist within the HSE tree with changes (typically small).
There are some external projects that we build as sub-repositories (in sub/). Licenses for sub-repos are documented in sub/LICENSES.md