What you need from him is a full confession and timeline. When, where, who, how long, and how far it went. How much detail as far as what all they got up to is up to you. Some people need all of the dirty details, and some only want to hear the basics.
As far as "why" goes, he's likely not going to have an answer that goes beyond blaming outside influences such as the state of the marriage, or some other form of blaming you or circumstances. This is very typical and common. So much so that we often talk about a mythical "cheaters handbook" that doesn't really exist, but the responses are so common as to be almost predictable.
Any reasons he gives that blame anything but himself and his selfish entitlement can only be a good reasons to have opened up and had an honest talk. There isn't a single issue that is solved or fixed by bringing a third party into your marriage. He should have turned to you and your marriage instead of someone else if he's having some sort of issues that have been bothering him. It takes time, digging, and lots of personal introspection for a WS (wayward spouse) to truly get to the root of their "why."
I'm not saying you had or have problems with your marriage, but those are the types of excuses we often see. Blame shifting is one of the first go-to moves for a cheater. Rare are the ones who come completely clean and take accountability right off the bat. It's also rare to get the full truth right off the bat. Brace yourself for TT (trickle truth) or more revelations down the line.
Another thing you need is for him to draft a NC (no contact) letter or message to his AP (affair partner). Preferably with your input and your approval upon reading it. Not some wishy-washy simple "I'm going to try and work on my marriage" message. More along the lines of:
"[Affair Partner's Name],
I need to be clear about my feelings and the situation. I’ve made unwise choices, and the truth is that I love [Your Name]. My marriage means everything to me and is my top priority.
Therefore, I must end all communication with you. I cannot see or talk to you again. This is non-negotiable, and I will be blocking your number and social media. This will be the last time you hear from me as I need to get my priorities straight.
[His Name]"
Or something similar. It needs to be definitive and final. Don't let him leave the door open for possible future communications.
Then he needs to follow through. No contact means NO CONTACT. He blocks her number and any social media profiles and goes completely no contact with her, never to speak to or see her again. If they work together this might include him finding another job. That might sound extreme, but what he did was extreme and you need to feel safe if there's any chance of R (reconciliation).
You should also have unfettered access to all of his electronic devices. Passwords, apps, etc. I know that sounds invasive, but he brought this on himself and you deserve some boundaries. This isn't about control. These are the conditions you want to require if he wants to R with you. He's a grown adult and can do what he wants, but if that involves infidelity and not making you feel safe, then he can be a big boy and have his cake, but not as your husband. This is you setting conditions and parameters for R. He can choose to agree, or he can show himself out the door. These are the consequences of his actions.
These suggestions are under the pretense that you even want to attempt R. If you decide to D (divorce) then it doesn't really matter what he does.
Stay strong, stick to your guns, and take your time making any permanent decisions. He can still do the above while you decide as a show of good faith, with the understanding it still might not be enough and you may still decide you can't do it. You're going to be on an emotional roller coaster for a while. Just know that's completely normal. Most of us have been on that ride, or are still currently on it. It sucks, but it's an unavoidable part of dealing with this trauma.