Documentation for the ports tree includes:
ports(7) https://man.openbsd.org/ports.7
packages(7) https://man.openbsd.org/packages.7
mirroring-ports(7) https://man.openbsd.org/mirroring-ports.7
library-specs(7) https://man.openbsd.org/library-specs.7
bsd.port.mk(5) https://man.openbsd.org/bsd.port.mk.5
bsd.port.arch.mk(5) https://man.openbsd.org/bsd.port.arch.mk.5
port-modules(5) https://man.openbsd.org/port-modules.5
And for bulk builds:
dpb(1) https://man.openbsd.org/dpb.1
bulk(8) https://man.openbsd.org/bulk.8
proot(1) https://man.openbsd.org/proot.1
Also see the OpenBSD Porter's Handbook
https://www.openbsd.org/faq/ports/
