My son hit 6 months old today, no idea where the time has gone, and can't believe how much I enjoy hugs and playtime with him. I'm so not a 'baby person', but he lights my life up so much in ways I didn't think possible

Also feeling pretty damn lucky to have been able to spend so much time watching him grow at this special time (working from home), and as a family too. Sort of not wanting it to end with going back to the office/looking at nurseries etc now. Which brings me onto something that's been confusing me loads recently...
I have a friend who's married with 2 young kids (2 and 4). He earns about 30k a year, no significant savings, kids are all fine, no disabilities etc, and they rent. She is a stay at home mum, don't think she's worked for 5 years or so really either, and apparently brings in an additional £650 per months on universal credits on top of his wage. But for the life of me I can't work out how...
I earn about 34k, and for the purpose of this question, have no significant savings and own my own house (we're buying imminently, so this that would be my 'savings' gone). We're all able bodied, no disabilities etc, and only claim the standard child benefit (about £21 a week) that anyone earning under 50k is entitled to. Every way I look at it, my partner is entitled to nothing other than this child benefit, and if we wanted her to stay off work for 3 years, I'd have to cover the house, bills, baby and all her needs etc.
So my question; Is there something significant my friend isn't telling me that allows his wife to bring in £650 a month even with his 30k wage? Or am I missing a massive part of the benefit system we could apply for so my partner doesn't have to go back to work and we wouldn't be living just off my wage? Anyone with experience on this I'd be keen to hear your thoughts, as it just doesn't add up to me!
(For the record, I have no intention of defrauding the system for money, nor do I expect the state to pay for my child. We made an active decision to have kids with the full knowledge I could, and would, be able to cover all money matters for the next few years at least. It's just confusing my brain trying to understand how she's claiming 650 a month and I need answers!)