I am reporting that I was able to resolve myself.
The key to solving this problem was the "character string".
I assumed that the content sent by P2P was not a "string" but a "number".
It may be hard to see, but I will attach the screen created by UIflow.
I'm an amateur, but I'm glad I worked hard to solve it.
(The attached image is the receiving Atom Matrix)

Introducing the UI Flow of M5 Stick C mentioned earlier.
Based on this, I am thinking of UIFlow for Atom Matrix.
I would be grateful if anyone had any good advice.

I want to synchronize the display of two Atom Matrix using UIflow.
What are some ways to achieve this with P2P?

One of the two is for transmission and the other one is for reception. Several types of LED patterns are programmed in the Matrix for data transmission in advance, and the LED display switches each time the A button is pressed. And I want the Matrix for data reception to display the same LED pattern as the Matrix for transmission.

Similar to the above has already been achieved with the M5 Stick Cs.

I also connected GoPlus 2 to M5 Basic.
Next, I connected Relay Units to PortB_1 and PortB_2 respectively and confirmed the operation.
UIFlow Block is based on the image posted by ajb2k3_san.
I didn't have to use a block of Relay Units.

@lgx
Thank you for your reply.
From your information, I knew that it was a Core 2 feature.
I want to use 4-Relay with my M5stickC.
I will continue to study UIflow.

Thanks for the great information.
And I have a question.
Is the block in the second half of the last figure a block for 4-Relay Unit?
My UIFlow (ver.1.7.3) doesn't have a 4-Relay Unit icon, so I can't select it.

I am suffering from a similar problem.
I tried to use Pa.HUB to use two Light sensors in addition to the M5 Stick C + UIflow.
However, even if unit can be selected, HUB does not appear in the menu for selecting Port.
Please let me know if there is a solution.