A message that my phone is infected by virus is displayed and a button appears to deactivate it now.Its not my antivirus anyway. Sometimes a countdown also begins. what is all this? is it really a virus?
Do not click the button, close the webpage instead.
Only your antivirus will alert you of viruses, this is obviously a scam to get you to click that button.
When you visit a website, your phone sends some information like which webbrowser it is using and the device model to the webserver, it is common that people abuse that by making a "personalized" message about virusses that is meant to trick you into going to a website of theirs or installing a malicious app.
They display the device model to make it look more real and to unknowing users this makes the people who made the message look trustworthy and allknowing, and the countdown is to put you under pressure.
I cannot tell you what happens when you do click the link because i never did that, but i can tell you that it is far more likely to install virusses than remove them.