First thing you must know is virus/Trojan must be an app.
Second thing to consider is, the virus can be any app even facebook (which is tempered by someone)
So first install an antivirus like Comodo Mobile Security from Playstore, scan and delete viruses. I prefer Comodo, because they have better Virus Search Engine.
Now go to Settings→Apps, Remove any app not installed from Playstore, if you see any app without icon (green android logo) in the installed app section, delete it too.
Now Go to Device Administrators, un-click any suspicious app (maybe an app without logo)
This is it, you are good to go,(all the above mentioned step are for unrooted phone, in a rooted phone the easiest thing you can do is to re-flashthe rom)
Generally, viruses for Android are uncommon. You are more likely to deal with malware that exploit the system to steal information or spy on you.
If you are doubtful, or your device is behaving strangely, then I recommend a full factory reset. Why take chances right?
Backup your data and then reset your device.
If you want something monitoring your device, then there are anti malware solutions like Comodo and antivirus solutions like Avast, Kaspersky, etc.
I hope this helps!
Nowadays Android devices have a malware problem and it’s growing every day.
If you do get a virus, you could perform a factory reset to get rid of it, but that means you’d lose all your data — those photos you shot, the saved games, the text messages, and everything else. Obviously, you want a factory reset to be your last option.
So what can you do to remove a virus from Android without a factory reset?
Check this link :- https://www.makeuseof.com/tag/remove-virus-android-without-factory-reset/
Removing virus from you smartphone is quite a tedious job to do, because it is embedded in your OS and you might even get frustrated. Android actually has a wide array of antivirus apps now, both free and paid, so the first step is to run an antivirus and see if that catches the malware. Delete or remove unwanted files from int. store and ext. store. Don't keep apps that u don't use. Scan your SD every week from your PC. If still present, then hard reset your phone.