Hmm. So when the 'cheating' happened, the two of you were quite new to seeing each other, were not 'official' and had not discussed or agreed being exclusive. In fact, you have never had the exclusivity talk?
In that case, why is he upset? Seriously, if neither of you has ever raised the subject, how is either of you supposed to know what the relationship ground rules even are? You can't just assume these things!
He seems to be expecting or even demanding behaviour from you which he has not openly committed to himself and that's not fair. He is also breaching boundaries be even asking about any past partners or experience. Have you interrogated him about his? Would he answer?
It's too late now as the lie popped out on self defence, but always remember you aren't obliged to submit to interrogation. Even the police allow someone a lawyer and you can say no comment!
It's best to turn a question like that back with another question, like, "Why would you ask me that?'' "Are we exclusive, then? When did that happen? Did we discuss exclusivity?" "Are you asking me to share details of my entire dating history? Why?" and perhaps most importantly, "Since you've raised the subject, would you like to go first?" It's always OK to respond "I don't think that's really any of your business, Jack!"
His responses to that will quickly show you how jealous and controlling he's likely to be and you can act accordingly.
So he had no right to question you in the first place. Having done that, he proceeded to invade your privacy by exploring your phone: a huge red flag right there! Nobody, regardless of relationship or gender, should be nosing about in anyone else's phone unless perhaps it's parent and child. We don't open other people's post or emails or read their diaries: this is the same. Anyone who does this is not respecting personal privacy and needs to to be shown the door.
If you do it, you need to stop and think.
You may have dodged a bullet with this man.
If you take him back or when you date someone else, for your own sake and theirs, don't allow anyone to behave as if they own you; remember your boundaries; discuss the relationship as you go without making or allowing assumptions to just grow like weeds; be clear about what you want; don't submit to bullying or interrogation.
Why even have a phone password if you share it? I'd use my thumbprint for a password unless I was a deep sleeper, of course! I hope you've secured your phone now!
We learn as we go and this has been a hard lesson. You're young and just starting out, so don't clutter your life with entitled people who already have a relationship blueprint ready for you. You have a right to discover who you are and what you want from a partner. Be choosy; you deserve to be happy and have the right to fire anyone who makes you miserable!