They usually find wrestling by clubs or older siblings, some by judo. When the Japanese wrestling federation first started the womens program, they recruited all judokas for the wrestling team. If you've ever wrestled against a really high level judoka, they have an extremely strong base and insane grip strength. I've paid really close attention to their program for a while and you'll notice a lot of their teams(cadet, junior, senior) have brothers on the same team or sisters. The siblings that rise to the top of their programs are usually pushing each other all of the time to get better.
A good example this year for the womens team is Yuki Irie at 50kg. She has a sister that was at 55kg. Both of them are former /cadet junior world champions and the first time representing Japan in the World Championships on the senior level. Both of them also had to beat multi-time senior world champions to get on the team. Another example on the womens team is Risako Kawai-this year's senior world champion at 57kg and her sister at 62 kg, Yukako Kawai(2nd in 2018 and 3rd this year at senior WCs). The most famous example are the Icho sisters. Kaori Icho- 4x Olympic champion, 10x senior world champion and her sister, Chiharu Icho- 2x Olympic silver and 3x senior world champion.
Another example is the Yamamoto family. If you know MMA, Kid Yamamoto's dad competed for Japan in the '72 Olympics. Kid's sisters wrestled as well-Miyuu Yamamoto(3x senior world champion, 1x silver) and Seiko Yamamoto(4x senior world champion). Seiko and Kid(Norifumi) went to high school in Arizona. Kid lived with Townsend and Tricia Saunders and won the state title 3 times, placed 3rd his freshman year. Kid placed 2nd in the Emperor's Cup back in 19989(Japan's world team qualifier) and 4th in 2007. He tried making a comeback for the 2008 Olympics, but lost a match where he dislocated his elbow in the Japanese trials.
The mens side also had brothers several times over the past few years, the Takatani brothers. Sohsuke Takatani(2014 world silver at 74kg) just lost to Myles Amine in the quarters 5-2 at 86kg. Daichi Takatani competes at 65kg, lost to Bajrang in the 65kg Asian Games finals last year, but has been kept off the team the past few years by Otoguro.
I don't think there is any one specific place that produces great wrestlers like Russia for example with Daegestan