Volker is correct to note that a trigger is not the best approach for this requirement. If you believe that this is required, the easiest trigger to manage this is a row level trigger. A statement level trigger has to account for the possiblity that the insert is not a single row insert i.e., insert into t1 select * from t2. That will require some additional work in the context of a statement level trigger - not much but it has to be handled. Here are examples (with limited validation but should generally work) of row and statement level triggers to handle what you described as output.

Row Level Trigger:

ALTER TRIGGER "tr_calc_sum_amts_for_id_row_level_trigger" AFTER INSERT
ORDER 1 ON "DBA"."t1"
REFERENCING NEW AS new_name
FOR EACH ROW
BEGIN
    declare sum_all_amts_for_id numeric;
    select sum( t1.all_amt ) into sum_all_amts_for_id from t1 
        where id = new_name.id;
    update t1 set remarks = 
        cast( sum_all_amts_for_id  as char(50));
END

Statement Level Trigger:

ALTER TRIGGER "tr_calc_sum_amts_for_id_statement_level_trigger" AFTER INSERT
ORDER 1 ON "DBA"."t1"
 REFERENCING NEW AS new_name
FOR EACH STATEMENT
BEGIN
    declare curs_ids cursor for 
       select distinct id from new_name;
    declare curr_id integer;
    declare sum_all_amts_for_id numeric;
    open curs_ids;
    lp: LOOP
    fetch next curs_ids INTO curr_id;
    if sqlcode <> 0  then leave lp  end if;
    select sum( t1.all_amt ) 
       into sum_all_amts_for_id from t1 
    where id = curr_id;
    update t1 set remarks = 
         cast( sum_all_amts_for_id  as char(50))
    where id = curr_id;
    end loop;
    close curs_ids;
END