This "slave" board is connected with the Master controller board. The "SimVim Matrix" firmware code of the slave board reads matrix grid state and sends it to the master board.
The "slave" controller board (Uno, Nano) is linked with the Master controller board using serial interface, when Tx output signal of one board is connected to the Rx iput of another board.
For the slave output control the Tx1,Rx1 port are used in SimVim. So, you should always connect the Tx pin (#1) of the slave board with Rx1 pin (#19) of the master board, and "slave" Rx (pin 0) - with the "master" Tx1 (pin #18).
Note: The same Tx1/Rx1 are used for SimVim Stepper board. If you need to connectc both Stepper and matrix boards you need to connect them "in-chain" (Master Tx - to the slave #1 Rx, slave #1 Tx - the slave #2 Rx, the slave #2 Tx - to the master Rx)
Note 1. Never forget to connect the GNDs of each board ( and other devices as well) to the common GND wire.
Note 2: When you need to upload Matrix board firmware to the slave board you need to disconnect the Rx,Tx wires before uploading. To simplify this you simply can add a "shut-off" switch breaking this two wires.
Another option - you can leave the Rx/Tx connected, but press the Reset button on the master board and hold it while uploading stepper firmware to the slave board.
Don't keep it connected to USB port after the SimVim Matrix firmware upload. You can use any +5V source connected to the slave board using USB connector, or +5V pin.