You need an `OPENJSON()` call with the appropriate `path` to access the nested `$.source_info.help_info.details` JSON array and an additional `APPLY` operator: 

Sample data:

    SELECT *
    INTO Data
    FROM (VALUES
       (1, 45745, '{"source":"XYZ",
         "source_info":{
             "type":"need_help",
             "help_info":{     
                    "questions":50, 
                    "details":[{"floatv":0.42,"name":"Tom"},{"floatv":1.08,"name":"Dick"},{"floatv":1.2,"name":"Harry"}]}}}')
    ) v (UID, ID, JSON)

Statement:

    SELECT d.ID, j.NAME
    FROM Data d
    CROSS APPLY OPENJSON(d.JSON, '$.source_info.help_info.details') WITH (
       NAME varchar(100) '$.name'
    ) j

Result:

|ID	|NAME|
|-|-|
|45745	|Tom|
|45745	|Dick|
|45745	|Harry|