how to add subcategories for different products

[beautyboulevar_]

 like how it’s one main category categorized into smaller ones

[thescriptflow]

It's a simple. You just need to create collection.
Here is the approach you can do. 

1. First you need to create a Main collection for example "Lashes" and in this Collection Put all Products.
2. After that Create Sub collections like "Glam Lashes", "Lash Vaults" and more. After that add products in these sub collections regarding their name.
3. After finishing this used them where you want. If you want to use in menu Just go to menu and link all theme Sub collections.