for (Account account : [SELECT Id, ... FROM Account ...]) {
   // Whatever
}

Database.getQueryLocator([SELECT Id, ... FROM Account ...]);

for(Account acc: [SELECT (SELECT Name FROM Contacts) FROM Account) {
  if(acc.Contacts.size() > 100) { // Do something
    ...

for(Account acc: [SELECT (SELECT Name FROM Contacts) FROM Account) {
  Integer totalContacts = 0;
  for(Contact[] con: acc.Contacts) {
    totalContacts += con.size();
  }
  if(totalContacts > 100) { // Do something
    ...

for(List<Account> accounts : [SELECT ID FROM Account]){
        for(Account acc : accounts){

        }
        System.debug(count);
    }