There are any strategy for sharding (auto-sharding)

heard that MongoDB has a auto sharding feature.
Here, I have a question. actually, i have two collection. one is just data model. and second is a meta data model for one.

like this.

LinkScrap {
    _id: "507f191e810c19729de860ea",
    items: [
        {id: 1, type: 1, title: "google", value: ""},
        {id: 2, type: 1, title: "youtube", value: "" }
LinkScrapMeta {
    _id: "507f191e810c19729de860eb",
    linkScrapId: "507f191e810c19729de860ea",
    userId: ... , 

two collection is very related. but if use sharded database then it can be fragment. There are any option to group two collection ? in auto sharding

and there are auto re balcing option in mongo DB ? and also there are same option (grouping)?