Random _id vs custom _id?

Hey there, I saw several topics about this point on the net but I never found exactly if I should prefer random or custom _id.
Is there anyone that can explain it to me? Furthermore, Is there any difference between a creation document and an update document using random or custom _id?

Really thanks

Indexing documents with an auto generated id is more efficient than using a custom id so is ideal for immutable data. If you want to update data you may benefit from a custom id as updating gets easier and you do not need to search for the id first.

Regarding update procedure. Using a custom _id is probably better I know from the beginning the document that I want to update, while with a random _id I should search it with a query? Am I right?

If you are going to update a lot it makes sense to sacrifice a bit of indexing speed in order to simplify updates.

In your opinion, executing a query (first operation) to get the random _id to be updated (second operation) is more expensive (cpu/jvm) than updating directly the document with the custom _id?

Yes. It therefore comes down to the ratio of inserts to updates and which is best to optimise for.