Oracle 18c

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create table t1 (id, vals) as
    select rownum, cast (null as varchar2 (32)) 
    from dual connect by level<=3
3 rows affected

create table t2 (id, t1id, val) as
    select rownum, a.id, 'val '||rownum from t1 a, t1
9 rows affected

declare
    cursor cur is
    select t1.rowid, listagg (t2.val,',') within group (order by t2.id) vals 
    from t1
    join t2 on t1id = t1.id
    group by t1.rowid;
    
    type currows is table of cur%rowtype;
    rows currows; 
begin 
    open cur;
    fetch cur bulk collect into rows;
     
    forall i in 1..rows.count
        update t1 set vals = rows(i).vals
        where t1.rowid = rows(i).rowid;
end;
/
1 rows affected

select * from t1   


ID
VALS


1
val 1,val 2,val 3

2
val 4,val 5,val 6

3
val 7,val 8,val 9