Item 17 in the diagram below
View attachment Check valve.pdf
There is also 1 or 2 under the primer bulb
There may be more than 1 check valve in the carb body itself depending upon which carb you have.
I can't be bothered to track down the parts breakdown for exact carb you have and you have not made the effort either or you would have found the parts break down showing the check valves.
It is on the Walbro web page along with the proper way to clean & diagnose their carbs.
They used to come in the overhaul kits. I have a couple of very old kits I inherited with the business.
The new kits do not have them but you can order them direct from Walbro, or Zama.
You can not get them for Ruzing or any other Chinese carbs as the only parts available for them are diaphragms & gaskets.
If you really want to know exactly how the carb works there is a really good series called "two stroke carburettors theory & operation" or something similar on You tube.
It is presented by an very well informed but utterly boring UK presenter.
Next best is the series by Joe Pace ( ex Husqvarna Service School ) called "Two stroke Diagnosis "
Taryl & Donny Boy might have some good videos but neither present much in the way of theory of operation.
Most everything else is just ego food for monkeys who watched some one make a video & post it then "monkey see - monkey do "