Why am always getting ads frequently on my phone

I have had the same problem on Ulefone, like I am on desktop and AD starts and after few seconds again. I took it to the service and they said that there is a virus on it. The repair was expensive, so I did it by myself. Firstly i restart it, then I deleted suspicious applications that I have not installed and turned off the ability to display through other aplications. After all of this, ADs were no longer shown on desktop. Hope this will help.