Retrieving list of documents from collection by id in nested list
Solution 1:
Solution 1
Use ElemMatch instead of AnyIn.
var filter = Builders<Example>.Filter.ElemMatch(
x => x.Family,
y => y.Countries.Any(z => z.uid == porduitId));
Output
Solution 2
If you are unconfident with MongoDB .Net Driver syntax, you can convert the query as BsonDocument via MongoDB Compass (Export to language feature).
var filter = new BsonDocument("Family.Countries.uid", porduitId);