Let me answer from Google's perspective.
What are the rules in 2021 for writing text for SEO?
Perhaps the following Google recommendations can help:
Content and quality questions Does the content provide original
information, reporting, research or analysis? Does the content provide
a substantial, complete or comprehensive description of the topic?
Does the content provide insightful analysis or interesting
information that is beyond obvious? If the content draws on other
sources, does it avoid simply copying or rewriting those sources and
instead provide substantial additional value and originality? Does the
headline and/or page title provide a descriptive, helpful summary of
the content? Does the headline and/or page title avoid being
exaggerating or shocking in nature? Is this the sort of page you'd
want to bookmark, share with a friend, or recommend? Would you expect
to see this content in or referenced by a printed magazine,
encyclopedia or book?
Expertise questions Does the content present
information in a way that makes you want to trust it, such as clear
sourcing, evidence of the expertise involved, background about the
author or the site that publishes it, such as through links to an
author page or a site's About page? If you researched the site
producing the content, would you come away with an impression that it
is well-trusted or widely-recognized as an authority on its topic? Is
this content written by an expert or enthusiast who demonstrably knows
the topic well? Is the content free from easily-verified factual
errors? Would you feel comfortable trusting this content for issues
relating to your money or your life?
Presentation and production questions
Is the content free from spelling or stylistic issues? Was
the content produced well, or does it appear sloppy or hastily
produced? Is the content mass-produced by or outsourced to a large
number of creators, or spread across a large network of sites, so that
individual pages or sites don't get as much attention or care? Does
the content have an excessive amount of ads that distract from or
interfere with the main content? Does content display well for mobile
devices when viewed on them?
Does the content provide substantial value when compared to other pages in search
results? Does the content seem to be serving the genuine interests of
visitors to the site or does it seem to exist solely by someone
attempting to guess what might rank well in search engines?
How much text is needed about a certain topic?
There are no explicit guidelines, requirements or guidelines for the size of the content.
How much should you repeat yourself? How does Google punish you if you
write too much of the same thing?
Check the guide Avoid creating duplicate content of Google.
An e-commerce platform has 5 shopping categories. Each category has
their own page with text. However on the front page they've chosen to
write paragraphs about each category. The text is a short summary but
not a copy-paste of the category pages. The text is accompanied by a
Is that good practice? To have small texts about each category all
over the start page? Or should you just have a little generic
paragraph about the whole site at the beginning?
In my humble opinion, your second point does not exclude the possibility of existence for the first point. The content of the home page by default has info about the content entities of the entire website including product categories. It may be pertinent to remember here that all websites offering online payment are treated by Google as Your Money Or Your Life - YMYL. Therefore, it is quite important to indicate in the content of the website comprehensive information about the organization of an e-commerce business, including the points of the business registration certificate. It is also important to provide complete information about the customer service and the person responsible for this service. In the content of the home page (before the fold), you can specify a description with the specifics of e-commerce and set a text link to a page with full information about the business. The same applies to customer service.