That's a feature that he made for his video. It's disabled by default, the only way to activate it is by going into the code of the game.
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I don't know what and how to edit the code, but i know how to get the code.
Go to this website:
https://github.com/SebLague/Digital-Logic-Sim
Then click on the green "Code" button and then click "Download zip".
Then go to your file explorer and right click on the file and click "Unzip".
Then, finally go into the folder to edit the code.
(I don't know what to change in the folder though).
That code as @fuighy mentioned, can be opened in Unity. I don't know though how to enable the display visualizer thing, but this a quote from another comment: "So, I found out that it must be somewhere in "the code" there is VidHelper.cs and VidHelper.cs.meta in there." If you do something with these files you can enable the features. And I know you can increase the size of the workspace, there's a guide here.
Assuming your using Windows, You can download Unity and there should be a button like 'open project' or 'open folder' then click the folder with the code in it, then Digital Logic Sim is open in Unity. If you have Notepad or Visual Studio (Code) then you can edit the code in there.
For a more in-depth explanation someone else in the community might know, I've personally never experimented with the code of Digital Logic Sim that much, just opened the code in Unity.