Well, it depends on the phone and the apps you have installed.
Some phones come with preinstalled apps that leave junk. It is quite annoying. One app that can show up preinstalled and cause this is called "Featured Apps" on some phones.
I suggest you go through the apps and check out their names online to see if they are indeed needed or not. Some may be unable to be removed (like the one I mentioned) but you can disable them.
If this doesn't resolve the problem, then you need to check the apps you have installed to see which ones have ad permissions on your device. If you go to "Settings" then "Apps", tap the app you want to check and scroll, you should see all the permissions it has.
Now should none of this help, then you may want to check the device for viruses, malware, etc.