with cte as ( -- your sample data:
select 1 EmployeeId, to_date('20.02.2020 12:15','dd.mm.yyyy HH24:MI') startDate, to_date('20.02.2020 21:15','dd.mm.yyyy HH24:MI') EndDate from dual
union all
select 2 EmployeeId, to_date('20.02.2020 13:15','dd.mm.yyyy HH24:MI') startDate, to_date('20.02.2020 22:15','dd.mm.yyyy HH24:MI') EndDate from dual
union all
select 3 EmployeeId, to_date('20.02.2020 17:15','dd.mm.yyyy HH24:MI') startDate, to_date('20.02.2020 23:15','dd.mm.yyyy HH24:MI') EndDate from dual
union all
select 4 EmployeeId, to_date('20.02.2020 20:15','dd.mm.yyyy HH24:MI') startDate, to_date('21.02.2020 01:15','dd.mm.yyyy HH24:MI') EndDate from dual
union all
select 5 EmployeeId, to_date('20.02.2020 01:15','dd.mm.yyyy HH24:MI') startDate, to_date('20.02.2020 12:00','dd.mm.yyyy HH24:MI') EndDate from dual
union all
select 6 EmployeeId, to_date('20.02.2020 00:15','dd.mm.yyyy HH24:MI') startDate, to_date('20.02.2020 06:00','dd.mm.yyyy HH24:MI') EndDate from dual
)
,time_points as (
   -- using UNPIVOT to get all time points from all intervals from both tables
   select distinct
      case when pt_type='STARTDATE' then pt else pt + interval '1' second end as pit
   from (select distinct EmployeeId,startDate, EndDate from cte)
   unpivot (
     pt for pt_type in (STARTDATE, ENDDATE)
   )
)
,intervals as (
   -- combining time points into intervals:
   select * 
   from (
      select 
        row_number()over(order by pit) rnk
       ,pit as startDate
       ,lead(pit)over(order by pit)-interval '1'second as EndDate
      from time_points
   )
   where EndDate is not null
)
select i.*, round((i.EndDAte-i.StartDate)*24*60) as e_interval_minutes
      ,v.*
from intervals i
    ,lateral(
        select 
           count(*) cnt
          ,listagg(EmployeeId,',')
            within group(order by rnk) r
        from cte c
        where i.startDate <= c.EndDate
          and i.EndDate   >= c.startDate
    ) v;
     
       RNK STARTDATE           ENDDATE             E_INTERVAL_MINUTES        CNT R
---------- ------------------- ------------------- ------------------ ---------- --------------------
         1 2020-02-20 00:15:00 2020-02-20 01:14:59                 60          1 6
         2 2020-02-20 01:15:00 2020-02-20 06:00:00                285          2 5,6
         3 2020-02-20 06:00:01 2020-02-20 12:00:00                360          1 5
         4 2020-02-20 12:00:01 2020-02-20 12:14:59                 15          0
         5 2020-02-20 12:15:00 2020-02-20 13:14:59                 60          1 1
         6 2020-02-20 13:15:00 2020-02-20 17:14:59                240          2 1,2
         7 2020-02-20 17:15:00 2020-02-20 20:14:59                180          3 1,2,3
         8 2020-02-20 20:15:00 2020-02-20 21:15:00                 60          4 1,2,3,4
         9 2020-02-20 21:15:01 2020-02-20 22:15:00                 60          3 2,3,4
        10 2020-02-20 22:15:01 2020-02-20 23:15:00                 60          2 3,4
        11 2020-02-20 23:15:01 2020-02-21 01:15:00                120          1 4

11 rows selected.