Think about what happens when you see an object. How do you recognize it?
Suppose someone gives you an orange. You look at it and instantly recognize what it is and how an orange tastes. That recognition is saññā.
Now, for that identification to happen, you must have seen and eaten oranges before.
Your past “seeing events of an orange is in your memory; it is in your “pile of past rupa” or rupakkhandha. Furthermore, you remember the taste of orange from your past experiences, i.e., the “taste of an orange” is in your saññākhandha.
Let us get that issue resolved first. Please read the following post and point out anything you don’t understand:
“Difference Between Physical Rūpa and Rūpakkhandha.”
If you understand, read the first six posts in that section and ask questions. Please quote the post and bullet # in question:
“The Five Aggregates (Pañcakkhandha)“