Tl;Dr: If your question is about an application that should be installed in your computer it might belong to Super User, if it is about an application that requires a web browser like Wikipedia and Youtube it might be ontopic in Web Applications while the question focus on the use of the application.
The best is that you read the on-topic page of each site
Additional, recently I have posted a
faq in Meta Web Applications
explaining the section of the site tour corresponding to the its scope: Welcome to Web Applications Stack Exchange.
Using Google Sheets, Wikipedia, Facebook, Gmail, Amazon and alike (application software) using a desktop web browser might be on-topic in Web Applications. This type of applications are known as web applications. They should not be confussed with desktop applications, mobile applications, IoT applications among others.
One exception about the questions that might be asked in Web Applications SE are troubleshooting like questions.
Using Excel, Zoom Meeting, Slack and alike (application software) not using a desktop web browser might be on-topic in Super User. This type of applications usually are referred as desktop application because they usually are accesed from the computer desktop.
Wordpress, Microsoft 365, Wayback Machine (archive.org), among others are specially tricky.
- Questions about using the through a web browser might be asked in Web Applications
- Questions about installing software, troubleshooing like questions might be on topic in Super User
- For other types of questions, please ask for a site recommendation in Meta Stack Exchange
There are some topics that might be specially tricky like spreadsheets as there are certain similarities and compatibilities i.e. functions, formula syntax among the different types i.e. web and destop applications.
The typical comparision is Excel vs Google Sheets but it also could be done Excel (desktop) versus Excel Online (web). When asking questions about spreadsheets please be clear about what application and platform.
When mentioning an application, in general, use the name giving by the application developer, i.e. avoid using "excel" as synonym of spreadsheet, or "messenger" as synonym of instant messaging / chat application, instead use Excel, Google Sheets, LibreOffice, Skype, Google Chat, etc.
Regarding the platform the most popular are:
The complete list of Stack Exchange sites is in https://stackexchange.com/sites.
If you still are struggling to decide what is the proper site where to ask a question consider to ask for a site recommendation in Meta Stack Exchange