I have multiple list in SharePoint that all feed into one large list. Sometimes a date will be added to these feeder lists but I can't get that date to update on the large list. Its seems this process relies on finding an ID number to find the right item on the list to update, but the only option I get on the update item step is for the SharePoint provided ID number on the feeder list. This does not work for me because that ID number is not the same on feeder list and large list. I have another column that provides a unique ID number that is consistent on all lists but it's not selectable under Dynamic Content. Do I have to use an expression for this? Where do I find the syntax for that.
EDIT based on new attempt: I'm having trouble expressing what I need. Let me try to outline what I am currently trying to do from the top based on the answer below.
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What I am trying to do: 1) An item is entered into the feeder list. This triggers a flow to also create the same item in the large list where all feeder list items go. 2) After the item is created in the large list, I then want to get the ID number from that list and update the "Parent ID" column in the feeder list with that number. 3) If an item in the feeder list is updated, I want to then use that ID number in Parent ID in the feeder list to find the item that needs to also be updated in the large list.
Problem: I am now stuck on step 2 because I don't understand something with ID's. After the item is created on the large list, I do a get item action to get that information from that new item on the large list. I then do an update item action. I specify that I want to update the feeder list and try to find the item to update with the feeder list ID. However, the ID it seems to pull to find the item to update on the feeder list is actually an ID from the large list, so it can't find the right item to update on the feeder list. If I just type in a number as the ID, the flow works, but the Parent ID from the large list does not update on the feeder list.
So, first question, How to I get the correct ID # from the feeder list so it finds the item it needs to update on the feeder list?