Will it be visible that the mailboxes are associated with the same app/smartphone?

Question

Mobile development

• The mail applications "Mail.ru", "Mymail" and "Gmail" have a feature that allows you to manage multiple mailboxes in one application.
  

• Also, the mail.ru mail service allows you to manage multiple mailboxes in one browser. (here is a description of this function https://help.mail.ru/mail-help/auth/multiauth)
  

Please answer the points:
1) Will the application developers (both mail.ru and mymail) / company employees (both mail.ru and mymail) see that the mailboxes are tied to the same application?
2) Will mail.ru employees see that the mailboxes are used in the same browser? (this is a question regarding this function: https://help.mail.ru/mail-help/auth/multiauth)

Answer 1

What is visible to mail.ru employees and application developers - only mail.ru employees and application developers know. And this can surprisingly differ from what they declare that they know ...
If you care about safety in the slightest, consider that everyone knows.