go to product >> select the product for which you wanted to add review>> open it you will a screen like this
in that screen can you the button <\> clk on that and write your custom code>> save it.
1: I would recommend using an app like Judge.me for example. Once you install it from the app store, they have an onboarding process to help you set it all up. You can then add review sections and blocks in the customizer.
2: In the app settings, they have a tab to "Manage Reviews", where you can also upload your own reviews using a csv file.