However, I do not understand the exact concept behind this. Interlinking both Data Types seem to have no effect on my project.
You don’t need to link the datatypes in both directions - it all depends on how you plan to access them on your app pages, but there’s no real disadvantage to linking them both ways if they are just a one-to-one relationship.
In your case (at least in the demo app you lined to above), your ‘light’ version datatype and the ‘heavier’ version are almost the same anyway - there’s only 3 more fields on the heavy one than the light one - so there’s really no significant advantage to splitting them into two datatypes.
But if your main datatype had, say, 50-60 fields, including some lists with maybe 20-50 items on each, then it would definitely be worth having an alternative ‘lighter’ version with only a handful of necessary fields to display, rather than loading ALL the data for every item on a page when you only need to show a couple of pieces of data, such as a name and price, for example.
The data of one Data Type don’t show up in the other data type. Or do I need to save the same information on both data types?
Yes, you’ll need to create and update the data on both datatypes whenever you create or update entries. So when you create one, you need to create the other and add the first one to it’s record to link it, then, if necessary, modify the first one to include the second one so they’re linked both ways (if that’s logical for your particular use case).
Once they are linked you can access the data from one by referencing the other: i.e. if your display page has an RG containing your ‘light’ display version of your datatype and you want your user’s to be taken to a page displaying ALL the data from the full datatype for that entry when clicked, you can navigate to the page, and send the current cells ‘light’ datatype’s ‘heavy’ datatype to the page.
That keeps your RGs loading faster as they’re only loading a small amount of data, but you can still access all the data from the other datatype wherever needed.
It takes a little bit more setup to make sure you always update both datatypes anytime you change them, but if your datatypes are heavy and you’re loading a lot of items on your pages it’s worth it to avoid slowing down your app.