No matter the flavour, documentation is a key part of the development process for the BSDs. Whether it is the Design & Implementation series which started with covering 4.3BSD in 1989 and more recently FreeBSD 10 in the fourth instalment of the series, or each projects own set of documentation. Documentation is important as it distinguishes intent & implementation as well as save a lot of question and answer emails. FreeBSD has handbooks, NetBSD has guides, OpenBSD has FAQs, DragonFly BSD has a handbook and all projects make their man pages available online as web pages. There is even a teaching course based around the The Design and Implementation of the FreeBSD Operating System, 2nd edition.
Each operating system release is a complete, self contained bundle, containing
the documentation and necessary toolchain required for building a copy of the
operating system from source.
cover the release build process for each operating system.
For the purpose of embedding the operating system it may not be desirable to build a full blown release. Depending on the choice of variant, either the functionality is built in as standard or a project exists to assist with generating customised images with ease.
NetBSD has a target for generating an image in build.sh, customisations controlled by variables set in mk.conf. FreeBSD had PicoBSD which is now superseded by NanoBSD. OpenBSD has flashrd and resflash. DragonFlyBSD has nrelease.