MongoDB is an [open source](https://whatis.techtarget.com/definition/open-source) database management system (DBMS) that uses a document-oriented database model which supports various forms of data. (From [here](https://searchdatamanagement.techtarget.com/definition/MongoDB))
For example, here’s how we can dissect an actual Object ID returned by an application: 5f2459ac9fa6dc2500314019
1. 5f2459ac: 1596217772 in decimal = Friday, 31 July 2020 17:49:32
2. 9fa6dc: Machine Identifier
3. 2500: Process ID
4. 314019: An incremental counter
Of the above elements, machine identifier will remain the same for as long as the database is running the same physical/virtual machine. Process ID will only change if the MongoDB process is restarted. Timestamp will be updated every second. The only challenge in guessing Object IDs by simply incrementing the counter and timestamp values, is the fact that Mongo DB generates Object IDs and assigns Object IDs at a system level.
The tool [https://github.com/andresriancho/mongo-objectid-predict](https://github.com/andresriancho/mongo-objectid-predict), given a starting Object ID (you can create an account and get a starting ID), it sends back about 1000 probable Object IDs that could have possibly been assigned to the next objects, so you just need to bruteforce them.