4.8UPDATE

Update means:

“I know which Salesforce record I'm changing.”

Usually you have:

Salesforce Id

Example:

[
    {
        Id: vars.accountId,
        Phone: vars.newPhone
    }
]

Then:

Update Account

Mentally:

Create
requires data for a new record

Update
requires existing Salesforce ID

Upsert
uses an external identifier to decide
insert versus update

Memorize that distinction.