Creating Unrelated Objects records using Standard REST API

Yes, we can create 2 or more unrelated objects also using Standard API in a single transaction.

If the request body includes objects of more than one type, they are processed as chunks. For example, if the incoming objects are {account1, account2, contact1, account3}, the request is processed in three chunks: {{account1, account2}, {contact1}, {account3}}.

A "single request" (in a single transaction), we can process 
1. up to 10 chunks.
2. up to 200 records.
3. No.of API calls consume per 200 records/10 chunks = 1 API Call (Call-in / Inbound call)

JSON: As per the below example, in my org, there is no relation exists between BOOK object and ACCOUNT object.

{
   "allOrNone" : false,
   "records" : [{
      "attributes" : {"type" : "Account"},
      "Name" : "example-Composite.com",
      "BillingCity" : "San Francisco"
   }, {
      "attributes" : {"type" : "Book1__c"},
      "Name" : "BOOK-Composite"
   }]
}

Similar to above, If you want to populate in junction object, Json Example is given in another stack exchange post,to refer CLICK HERE

tHiNk gooD and dO thE bEsT.........MANJU NATH 🌝