Assuming you've watched Sebastian's video you know how they work, but the ability to see the display from outside a chip , like shown in Sebastian's video is not possible, normally, in the game, so here is a very long-winded and over-sized post about this subject and the greater Modding and Experimental Features of DLS (Digital Logic Sim).
Explanation:
By default this feature is disabled, turned off before Sebastian releases a new version of DLS, but according to Redcraft7578 and his video "How to enable chip displays in DLS" it is enabled as follows: (Digital-Logic-Sim > Assets > Modules > Chip Creation > Settings > Chip Overrides > "Enable Script Functions"), though directly in the video this is shown to not work perfectly.
A Note: there is VidHelper.cs and VidHelper.cs.meta, but this are, currently, not known to be connected to the experimental features.
Other Developments:
This post has gone from how to enable it, to the history of enabling it, to now a completely different subject, if you can't tell already I talk a lot. But this is just some other developments I'd like to mention which might eventually lead to the ability to enable this, and many other already implemented, and custom features, Modding Support is being added by Chazzwazzler first mentioned in this post, and also linked is a mod for expanding the Workspace in DLS.
See the comment by TekraGit under neither this post. They were able to get a working version of the 7-Segment Displays in DSL with their mod: here.
Epilogue:
Thanks for reading by very long random post, and I hope this developments in the DLS community evolve into a much larger project.