Default tax is listed in a few sections in Hike. Below is a brief explanation of how each of them works.
The default tax for an outlet can be selected from Store setup > Outlets and registers > Click on the name of the outlet > Default tax.
After selecting the default tax here, it will be applied by default when you create new products in that outlet. Default tax selected here will also be applied by default for the custom sales in the outlet. If you have different default taxes for different outlets and keep pricing different between outlets, the default tax of each outlet will be applied to the product when you create it.
You can still change the purchase tax in the purchase order. Select your tax rate from the drop-down list if you want to change the tax in it.