Keep two of my tools in sync

Keep [source tool, e.g. my CRM, a Google Sheet, Airtable, a Trello or Monday board, my calendar] in sync with [destination tool] for [which records, e.g. contacts marked Active, deals in stage Closed Won, events tagged Client]. Open the source and list every record that matches the rule. For each one, check whether it already exists in the destination by [matching key, e.g. email, company domain, event title and date]. Create the ones that are missing and update the fields that changed: [field mapping, e.g. name -> Name, stage -> Status]. Never create duplicates and never delete anything in either tool. If a record is missing data the destination requires, skip it and put it on an exceptions list instead of guessing. Finish with a change log: created, updated, skipped with reasons, and the total now in sync. Say if any field mapping looked wrong.