ftp://ftp.freebsd.org/pub/FreeBSD/releases/ISO-IMAGES/11.0/
crypto signature and downloading the checksums from the same (possibly
They're in the release announcement linked in OP. As I've said before, FreeBSD has issues with strong cryptographic provenance, stemming from their choice of repo, on out to iso's and packages. But they're getting better fast. ie: They're almost, if not 100%, reproducible builds now... see new flags to ar(1) for a simple example, commitlogs 'reproducible'.
As an aside, why big vendors choose linux (android, wireless routers, etc) instead of the permissive BSD license (do the fuck what you want, no GPL, no Stallman)? (BSD appears to support less hardware, but for few bucks this can be solved).
Vendors are cheap, including not paying devs to "do the fuck they want", so they choose whatever licence won't get them sued (either is fine), and whatever os has been cobbled together for their hardware, and is known to the vendors cobbled together team. That's usually linux, or windows.