You can seperate products in ad groups using subdivisions. Subdivisions are categories, product type, brand, item id and custom labels.
Simply choose a subdivsion. If you have custom subdivisions I recommend adding custom labels.
That is correct and this is how it works. You can only subdivide one level at a time and only choose one type of subdivision. Each level you can change the sudivision if its available.
My understanding is that you are trying to subdivide in a way that is not possible.
If you explain in detail how you want to subdivide, then I may be able to guide you.
With regards to your screenshot, simply add the data in your data feed supplier. If you are using Shopify, then add the data in Shopify google shopping app on a product by product level..
For the missing attributes in your feed - these can be populated only when using the native Shopify Google Shopping app. There should be an option (in the app and on your products) to "Edit Google Shopping fields"
Subdivisions, especially in Smart Shopping Campaigns can only ever be handled via top-level groups such as Brand, Category, Custom Labels even. If there is no other or limited number of subdivision options beyond the the first level then it means these attributes either don't exist or are incomplete in your feed.
For Smart Shopping Campaigns it is highly recommended to only use one top level subdivison (ie BRAND) and then to allow Google's Machine Learning to do the rest of the work.
If you mean, for each individual product, no, there is no function for that. You will need to dig into Google Ads scripting for that, which is pretty complicated. You will need to ask that question here: https://groups.google.com/forum/#!forum/adwords-scripts
As I have been managing clients over 10 years now, I recommend you group them by brand or product type, and or if you have different bidding amounts, to choose custom labels to separate them.
Then when you add a new product, and if you have created ad groups using the subdivision levels (not with item id) then it is all automated.
If you want to subdivide down to item id, you need to go with scripts.