2 of my boxes have a DE-9 RS-232 COMmunication port.
https://en.wikipedia.org/wiki/Serial_port

I heard you can use COM to control an OpenBSD machine.
What do I need for it?

Do I understand it correctly (from https://youtu.be/O1MhwPb2NWA ) :

On a COM-equipped machine, as per:
https://www.cyberciti.biz/faq/openbsd-connect-serial-console/
https://marc.info/?l=openbsd-misc&m=159870241828469&w=2

cat /etc/boot.conf
set tty com0
stty com0 115200

Reboot of course.
Or type in boot prompt:

set tty com0
stty com0 115200
boot

Acquire a female COM to male USB adapter.

This one from wikipedia link above has a built-in processor and requires a driver.

This one has "No converting chip inside" and is "Directly connecting"

Which one do I need?
Of course I need the adapter to have female COM.
The question is about the converting chip and driver.

Connect COM-machine to a laptop via the adapter.
On a laptop, what do I do?
The cyberciti link above: cu -l /dev/cua00 -s 115200.
But doesn't it expect the laptop to be connected to another device COM to COM?
How to make it work with USB to COM?

Additionally:

Is COM console-only experience or can you somehow do minimal GUI?
https://marc.info/?l=openbsd-misc&m=159872088031897&w=2

if you have FDE, the mount will hang, as there's no way to enter the password, without a serial cable. "set tty com0" will tell init to read from the serial, not the physical keyboard

https://marc.info/?l=openbsd-misc&m=159873917103029&w=2

if you have a monitor and keyboard attached you will no longer be able to use them

so COM xor keyboard+monitor?
Is there way to use PS/2 or USB keyboard and then use COM instead of a monitor in case you need to see output?

A machine with no COM - can you connect it in a COM fashion with a converter like depicted above, or directly via USB?
Why almost all ports (USB, PS/2, Parallel, Jack, Tulip, AC etc.) are female on devices and male on cables whereas COM (and some power supply) ports are male on devices and female on cables?