Normally, the isn’t a dedicated FW for right&left. There is just one FW for both and you flash that to both halves. The KB determines which side is right and which is left by either the half the USB cable is connected to ( e.g. FW is configured with “USB half is right side”) or by flashing a small marker to the EEPROM of the controllers to mark them as right/left (this marker us not part of the FW, look for EE_HANDS in the QMK docs).
Flashing with the .uf2 file provided by the webpage, you should get the default behavior of the keyboard and it should generally be working.
If you want to change any behavior (like key layout or anything) you would need to flash the halves with your own config of QMK anyways. That is kind of the neat thing about custom KBs - you can configure your device to your liking.
If you need some inspiration on existing boards: golem.hu/board/ --> set the filter on “ortholinear” for “matrix style” boards.
First you need to find a board, then we can help you to build it.
(You COULD build a board from scratch, but if you have not built a board yet, or have no background in mech. / electr. engineering, this me a bit too much at first.)