As mentioned in the Introduction and Assignment article, only the data elements can modify your records. You can think of the actions inside the flow as editing the records, and “Update Records” as clicking save. Similar to other data elements there are also two ways to update records:
1. Use a Record or a Record Collection
Commonly used when you have record or record collection variables that allow for input and you modify those variables in the flow. For the collection variable, it will often be combined with a loop.
2. Specify Conditions and Set Fields Individually
If you have a Screen element that allows users to enter new values, it might be redundant to update the record variables because the system will automatically create a new variable for each screen component. In this case, it is recommended to use this method. However, please note that this will cost an extra SOQL query.
By setting the fields individually, you can either update the fields using another resource, or a manually typed in value.
3. Use The Reocrd That Triggered This Flow (Only in Record- or Schedule-Triggered Flow)
This is similar to option 2, but it DOES NOT cost an extra SOQL query. Now for before-save triggered flow, you can choose to use an Assignment, or an Update Records with this option to update your records.
Check Out How Record Id Can Be Used In Real Cases!