Basically, the original study looked at Jewish kids in Israel versus the UK and saw that peanut allergies were about 10x lower in Israel, even though Jews of European background (Ashkenazi) there are fairly similar, genetically, to Ashkenazi Jews in the UK (the majority of both groups migrated recently from Central and Eastern Europe).
There is a snack called Bamba that they eat in Israel. It's kind of like a peanut butter Cheeto puff, and it is a nearly-universal snack for young kids in Israel. It melts in your mouth, so a 6-month-old can eat it almost as soon as they eat solid foods. It's about as popular in Israel as Cheerios are in the US, maybe more so. The hypothesis is that Bamba consumption there dramatically lowers the risk of developing peanut allergies.
[1] https://www.webofscience.com/wos/woscc/full-record/WOS:00026...