A company wants to build an Adobe Commerce website to sell their products to customers...

According to the Adobe Commerce documentation, the best way to add a custom tax calculation to all orders is to declare a new total collector in the “etc/sales.xml” file of a custom module. This way, the custom tax logic can be implemented in a separate class that extends the \Magento\Quote\Model\Quote\Address\Total\AbstractTotal class and overrides the collect() and fetch() methods. The collect() method is responsible for calculating the tax amount and adding it to the quote address, while the fetch() method is responsible for displaying the tax amount in the cart and checkout pages. The new total collector can be assigned to any area of the order totals, such as before or after the subtotal, shipping, or grand total.

References:

Customizing order totals

How to add custom fee or discount to order totals in Magento 2