Tabs keeps opening, claiming to be apple support and telling me I have a virus

(My apologies if I put this in the wrong section/community, this is my first time and I do not know which is best for this question.)

I use a MacBook Pro, Mac OS High Sierra Version 10.13.4, processor 2.3 GHz Intel Core i5, memory 8 GB 2133 MHz LPDDR3, graphics Intel Iris Plus Graphics 640 1536 MB.

My Safari is a Version 11.1

I do not entirely know when this problem exactly started, but the first time I realised was yesterday. Checking my history, the fake Apple Support website didn't pop up anytime throughout this week but yesterday and today.

I've read a discussion about what I think is the same problem I have, but it did not help me resolve my problem as I have not got a free Mac protection software like the user did. fake apple support keeps popping up

Randomly I had found Apple Support within my open tabs, which I knew I didn't open. I clicked to see what it was, and it was a site claiming to be Apple Support (however definitely fake) and telling me I had three viruses on my Mac, showing me a countdown of two minutes within I should call Apple Support for help. I immediately closed the tab as I feared I would get some sort of malware on my Mac, which I believe I did not have in the first place. I cleared my history and tried finding the website in my cookie list to remove anything it may have left behind, but I failed to find anything under the name which this website used. I then dismissed the issue thinking it was a scam site trying to get me to download malware, and after closing nothing happened until this morning. Once again the tab popped up, however this time it popped up when I was downloading a file (for personal reasons) from Picosong. I was confused as to why the tab popped up at the exact same time I downloaded the file. Also, this was the first time I used Picosong on this MacBook; I had used it on another one of my Macs before and never got any malware, trusting the site. I reacted the same way by closing the fake Apple Support tab. I know it was the exact same fake site, it looked identical with the same countdown and telling me I had the same viruses. I was worried, however, that it would mean what I did yesterday wasn't effective. I went to check my history once again and, lord knows how, my entire history from today was FILLED with about hundreds of "Apple Support" when I only saw the tab pop up once. I went on to do the same thing as the other day and removed my history, hoping it would be the slightest effective.

I don't know whether this might contribute to the issue, but I rarely fully shut down my MacBook. I always shut the lid and put it to sleep when I want to stop using it, and recharge it when it goes down to five percent so it doesn't close down then either. I've had this MacBook Pro since late March if thats any helpful information. I have not downloaded any Mac protection softwares as I know they can be harmful, but have downloaded a painting software (Which is safety approved by apple) and Roblox (Also safety approved by apple.) I haven't gotten these programmes from any other place but the official websites and didn't get any malware with them, however I do often read articles on various websites if that is any helpful information. Is it possible I could've gotten some sort of malware from one of those article sites?

I would also like to point out that I highly, highly doubt Picosong has anything to do with this issue. This issue started before I used Picosong, and the tab only opened when I downloaded the file. I would also like to say that I removed the file I downloaded shortly afterwards and searched my desktop and downloads for anything dodgy, but failed to find anything unwanted.

My point is, unlike in the other discussion, I didn't install any kind of Mac protection, as I knew Macs were able to take care of themselves, which was the cause to this problem for many other Mac users. It is why I am confused as to what the root of this problem may be and how i'm supposed to act when I don't have any Mac protection software to uninstall or suspect of causing the problem. How should I react if the tab comes up again later? How can I resolve this issue? How do I find the root of the issue?

I will try shutting down my MacBook and hope it will prove to be effective in resolving the issue. So far I have found no temporary fix for this.

Many, many thanks to the people who reply.