Yes it is a requirement for ALL files types.
JS/CSS commenting is helpful but basic guidelines on how/what to edit esp. key things like logo, editing images, adjusting CSS etc.
Without seeing the file in question it is hard to give detail.
There’s a template here https://help.market.envato.com/hc/en-us/articles/202500894-Helpful-Resources-for-Authors
When you download the “Installable WP file only” you are choosing to “save” it not “open” it right? I just did it with this theme and it comes out as a zip.
Alternatively if you have this folder just zip it up again and then install it.
If you choose to download “All files and documentation” then in here is: Documentation, enfold.zip, images, Licensing, pdsfiles.zip
You need the enfold.zip file to upload as it is or UNZIP it and you can use FTP
Best practice design esp on the fundamentals like typography, alignment, spacing etc. (without seeing a design it is impossible to advise on this).
Take a look at other files for a guide on quality and standards
If it is WordPress then it needs to meet these https://help.market.envato.com/hc/en-us/articles/202822450-WordPress-Theme-Submission-Requirements
The WORST thing you can possible do is rush or see submitting files here as a quick fix payment. Success here takes time and commitment.