SSIS - merge table and split transformation function

i would like to ask if i have to update the existing record or insert new record from SSIS.

here is my step in data flow control:

1. create a source table A and table B

2. merge two tables by left join A to B with ID,NAME,TEL column

3.add split transformation control and then set condition as belows:

   3a) output name: insert row   ; condition: isnull(ID)

   3b) output name: update row ; condition !isnull(ID)

4. route to different OLD DB destination  (one is update, another is insert row)

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but i don't understand the step 2 and step 3. when i left join table A and B by ID,NAME,TEL and then check isnull(ID)/!isnull(ID) in step 4 from the result set of merge join.

for step 3, is it only check if the result set of merge table's "ID" column is null then route to the step to Insert row. not check the join key from step 2?