The video explains the typical connection of each MIDI connector. Nowadays you can find only USB connectors on many synthesizers, but if you need to connect individual devices, the classic connector has been doing this for many years. The video focuses mainly on Behringer synthesizers.
If you need to connect more synthesizers, then you connect everything to MIDI Out first. Next, you proceed to MIDI In and the output goes to MIDI Thru. And there you connect all synthesizers in series. Each synthesizer needs to be assigned its own MIDI channel. The first synthesizer should be capable of sequencing all other instruments in series.