Apex Bulkification: The Only Pattern You Need to Memorize

// The Collect-Query-Map-Process-DML pattern:

public class OpportunityTriggerHandler {
  public static void afterUpdate(
    List<Opportunity> newList,
    Map<Id, Opportunity> oldMap
  ) {
    // 1. COLLECT — gather what you need
    Set<Id> accountIds = new Set<Id>();
    for (Opportunity opp : newList) {
      if (opp.StageName != oldMap.get(opp.Id).StageName) {
        accountIds.add(opp.AccountId);
      }
    }
    if (accountIds.isEmpty()) return; // exit early

    // 2. QUERY — one SOQL, not N
    Map<Id, Account> accountMap = new Map<Id, Account>(
      [SELECT Id, Last_Opp_Stage_Change__c
       FROM Account WHERE Id IN :accountIds]
    );

    // 3. MAP — already done (Map<Id, Account>)

    // 4. PROCESS — enrich using the map
    List<Account> toUpdate = new List<Account>();
    for (Opportunity opp : newList) {
      if (opp.StageName != oldMap.get(opp.Id).StageName) {
        Account acc = accountMap.get(opp.AccountId);
        if (acc != null) {
          acc.Last_Opp_Stage_Change__c = System.now();
          toUpdate.add(acc);
        }
      }
    }

    // 5. DML — one operation, not N
    if (!toUpdate.isEmpty()) {
      update toUpdate;
    }
  }
}