Taxes you've defined in the settings section can be selected for each Product & Service you create. If a tax is selected for a product or service, the tax will be automatically applied when the item is added to the invoice. Taxes can also be selected manually for any items added to an invoice.
You can create taxes on the fly while setting up Products & Services. To manage taxes, go to Settings > Billing Hub > Taxes
Note: If you are syncing transactions to Quick Books online you will only be able to select one tax per Product and Service Item.