Hey there, I have a question about the range index. Are Pinot allowed to have multiple range index?

Yes, you can have multiple columns (fields) with range indexes

Then I am wondering, if the column is not sorted physically, how could we build the range index? Is this index more like a nry-tree, hash table or SSTable structure? How can we store the actual document id as the value of a specific range?

List of all doc id that belong to a range

See https://docs.pinot.apache.org/basics/indexing/range-index