How are frontend and backend combined?

Question

Web development

Interested in how applications are developed that are written, for example, in ReactJS and backend in NodeJS or ASP.NET.
For example, there is an online store that sells confectionery chocolate. And for example, on the first page there is a list of all available flavors / types, and when the user goes to the page of a particular type of chocolate:
1) Should there be server-side routing or should react-route be used?
2) In what cases server rendering of the page will be better than front rendering?
3) Where should be the login / registration logic for the site (more precisely, the login logic for the site, where is it better to implement on the front or on the back?)
4) The third question, if the information is stored, for example, in the mysql subd. How should the information be pulled out from the front or from the back?
5) If there is a payment (transaction), should it be processed through the same back or maybe on the front?

Answer 1

2. Server rendering is better because reduced customer requirements. Client-side rendering puts a lot of strain on the visitor's browser.
3. Partial check of fields on the front, the rest is all logic plus duplication of checks of all fields on the server
4. Of course, back. If you do this on the client, then say goodbye to your entire base. The first script-kiddy will break you. Although there is nothing to break here, everything is open.
5. See point 4.
The advantage must be given to the back. This is the core, the heart of the project. The requirements for the back are stricter, there are more severe guys here.
The front is just a pretty wrapper. Here come those who need beauty, ruffles, whistles and fakes.

Answer 2

Communication between the front and the back happens via API
2. SEO
3. The login form is obvious on the front, the data is validated on the back
4. How do you pull information from MySQL through the front? Backend only
5. Backend