Thank you for your response.The web site was among those listed when I did a search on Google. The pop-up appeared when I tapped on the URL to go to that site.
You're welcome.
If a popup appears unsolicited, I would advise just to close it and never click it as you have no idea where it is going to take you.
Having done a search you should just get a list of matching results. If a popup occurs when you click that link a popup appears, there is some underlying redirection going on (commonly ads, but could be a malicious redirect).
The fact that Avast alerted when you clicked the link means Avast has prevented you connecting to that link/site and preventing anything being downloaded to your mobile. So you should in theory be OK, having run some scan appears to confirm this as nothing was found.