How to filter data by orderByChild()? [duplicate]

Question

I want to filter data and put it into recyclerview accroding to it's category . And any Item can have multiple category.

My firebase hierarchy follows

My code is

private void processFilter() {

    ArrayList<Items_modal_for_adp> list2=new ArrayList<>();
    ArrayList<Items_modal_for_adp> listKey2=new ArrayList<>();

    database.getReference().child("Items").orderByChild("Cat").equalTo("Fitness").addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(@NonNull DataSnapshot snapshot) {
            list2.clear();
            for(DataSnapshot snapshot1 : snapshot.getChildren())
            {
                Items_modal_for_adp adp=snapshot1.getValue(Items_modal_for_adp.class);
                list2.add(adp);
            }
            listKey2.clear();
            for(DataSnapshot snapshot2 : snapshot.getChildren())
            {
                Items_modal_for_adp adp=new Items_modal_for_adp(snapshot2.getKey());

                listKey2.add(adp);
            }

            adapters.notifyDataSetChanged();
        }

        @Override
        public void onCancelled(@NonNull DatabaseError error) {
        }
    });

    adapters=new itemAdapters(list2,listKey2,getContext());

    binding.recyclerUser.setAdapter(adapters);
}

I don't have a good knowledge of " orderByChild()" and how it works.

Can anyone clear my doubt?