How can custom fields be added to the product entity?
For example, it is required to add some fields such as arrival date, preorder status, preorder % amount.
There is no native product data entity in Master Data. You would have to create it from scratch. I would do that by simply adding the Product or SKU ID as one of the fields in the data entity, alongside the other fields you mentioned.
This quick example from our Learning Center might help you understand the modus operandi:
You can store documents in Master Data V2 without any predefined schema, but doing so allows you to search and/or filter documents based off specific fields. This documentation explains the schema lifecycle, so you get the full picture: