Insert Records in multiple Objects in single DML | https://www.mhamzas.com/blog/2020/04/06/insert-records-in-multiple-objects-in-single-dml/

singledml.cls

List<sObject> records = new List<sObject>();
records.add(new Account(AccountExternalId__c='1234555', Name='ABCD Company'));

records.add(new Contact(account=new Account(AccountExternalId__c='1234555'),lastname = 'Testlast', ContactExternalId__c='3525'));

records.add(new AccountContact__c (Contact__r = new Contact(ContactExternalId__c='3525')));

insert records;