Welcome to the forum!
I’m sure there are videos somewhere but I can describe the basics here.
The Digitakt is designed to be controlled by MIDI Control Change (CC) messages. A list of the sound parameters and the associated CC messages is given in Appendix A of the Digitakt’s manual.
To send a message to a specific audio track on the DT, each of the 8 tracks can be set to receive on a different MIDI channel number.
The MIDI channel numbers for tracks 1 to 8 are set in the DT’s SETTINGS > MIDI CONFIG > CHANNELS menu. It seems reasonable to set channel numbers 1 to 8 for tracks 1 to 8 respectively.
You need to ensure that the DT is responding to the CC messages by activating RECEIVE CC/NRPN in the SETTINGS > MIDI CONFIG > PORT CONFIG menu.
Therefore, as an example, if you wanted to control the filter frequency on track 4 of the DT, you need to send it MIDI CC message 74 on MIDI channel 4.