minadbd is analogous to the regular adbd, but providing the minimal services to support recovery-specific use cases. Generally speaking, adbd = libadbd + libadbd_services, whereas minadbd = libadbd + libminadbd_services.
Although both modules may be installed into the recovery image, only one of them, or none, can be active at any given time.
The start / stop of adbd is managed via system property sys.usb.config, when setting to adb or none respectively. Upon starting recovery mode, adbd is started in debuggable builds by default; otherwise adbd will stay off at all times in user builds. See the triggers in bootable/recovery/etc/init.rc.
minadbd is started by recovery as needed.
minadbd, recovery stops adbd first, if it's running; it then forks and execs minadbd in a separate process.minadbd talks to host-side adb server to get user requests.minadbd handles some requests directly, e.g. querying device properties for rescue service.minadbd communicates with recovery to fulfill requests regarding package installation. See the comments in bootable/recovery/install/adb_install.cpp for the IPC protocol between recovery and minadbd.minadbd, recovery restarts adbd if it was previously running.