passing the dynamically created file to notebook

Firstly, in order to avoid race conditions on using utcNow in multiple places and getting different answers, I'd probably create a pipeline variable for the filename and 'Set Variable activity' it to be @concat('user_1', utcNow(), '.txt')
Then I'd use that variable as both the destination filename *and* a parameter in the Notebook activity.  And then in the Notebook I'd have a parameter cell that uses the filename parameter.
So:
Pipeline

Variable setup

Set Variable activity

Copy Data Activity destination

Notebook activity

And the notebook itself

If this helps, please consider Accepting as a Solution so others can find it more easily.