Magento – Magento 2 Import categories using csv

This is not how you import multiple categories. The correct way is to add a new line but without sku:

sku,name,_category
1,Product 1,Category1
,,Category2
,,Category3

As long as there is no new SKU, all lines belong to the same product.

Not using any extensions is limiting you, but you could use Import/Export with products because missing categories will be created when you import products. Afterwards, delete the created products.

Depending on your number of products and categories, it might be easier to create a single dummy product, assign it to all categories and then export only this product.

Note: This will not export any settings of the categories, so it's a workaround, not a full solution.