Feature updates contain base updates and additional updates.
Cumulative updates only have base updates.
Generally feature updates are recommended for generating ISO files.
Enterprise is a virtual edition of Windows 10. "Virtual" here does not mean "for virtualization", but rather "not standalone".
Virtual editions require a "base edition" for conversion.
When selecting the edition for a given update, on the right side of the page you will find a table with requirements for different virtual editions.
For "Windows 10 Enterprise" the requirement is "Windows 10 Pro", which is what you will need to select first.
After selecting Pro click "Next" and choose the "convert and create additional editions" option.
A list with all possible virtual editions for your selection will appear;
From there you can choose "Enterprise" and create a download package.
Windows Server Updates only have Feature on Demand packages.
You cannot use them for conversion purposes.
They are only listed for easy access to Feature on Demand packages.
You can't. The conversion will fail if you do this.
In general, this means that the build was pushed to more than one channel.
For example, a build was first pushed into the Dev channel and then pushed into the Beta channel.
It does not matter which one you choose, they have absolutely the same content.
You can join UUP dump discord server and ask your question there. Only questions in English will be answered.