I don’t know if there is no optimization in the index village. There is no in-depth understanding of indexes. Please help explain it by the way!

1. Each piece of data has geographical information. (province, city), then I created an index:

db.v.createIndex({city:1}
db.v.createIndex({area:1}

My doubt is, when this is created, is there no piece of data to create an index value or is it created based on the index value?

    比如存在这样的数据
    id name         city         area 
    1  zhangsan     beijing      beijing
    2  lisi         shanghai     shanghai
    3  zhaowu       beijing      beijing

   #每条数据建立 
    city->beijing->1
    city->shanghai -> 2
    city->beijing->3
    
    #还是根据索引值
    city->beijing--|-> 1
                   |-> 3
    city->shanghai->2
    
    

Thanks