In your case, it seems like creating a formula field might be better for you instead of an automation. Formulas will always calculate for all of your records, without you needing to worry about updating individual records.
However, if you do stick with automations:
Automations will only work moving forward AFTER you create the automation — they won’t go back in time and update older records that are already in the system, as long as those records remain static & unchanged. But if those older records get changed (and the change matches the condition of your automation’s trigger), then the automation would trigger for those records.
Additionally, the “Find A Record” function doesn’t work in the way that any of us would expect it to work — sadly, it doesn’t actually go through all the records that were found afterwards.