Map

Map with List by Key

✅

Map<String, List<SObject>> industryToAccounts = new Map<String, List<SObject>>();

for (Account acc : [SELECT Id, Industry FROM Account]) {
    if (!industryToAccounts.containsKey(acc.Industry)) {
        industryToAccounts.put(acc.Industry, new List<SObject>());
    }

    industryToAccounts.get(acc.Industry).put(acc);
}