By default an Agent just wants to execute jobs in a Docker container, using the docker backend. But there are multiple backends. To run a build on Windows/MacOS, you would download the Go binary for the Agent for that platform, and execute it with the right environment variables to specify the local backend.
I think a lot of this functionality is just now getting added to Woodpecker so it's not been documented. It's been in Drone for a long time, but the fork is from a while ago.
https://woodpecker-ci.org/docs/administration/setup https://woodpecker-ci.org/docs/administration/agent-config https://woodpecker-ci.org/docs/usage/pipeline-syntax#platfor... https://github.com/woodpecker-ci/woodpecker/tree/master/pipe...
And I'm not saying that not supporting yet is a problem, but the docs saying "everything happens in Docker" implies a lack of forward-thinking in the design of the system that wholly excludes working elsewhere (including FreeBSD or other FOSS platforms).
Apparently it is being supported, but the docs will need some clarification now that some of the advertised assumptions are no longer going to be true.