How do I finish up in TurboTax after filing an extension? I don't see the option to go back to my return.

If you used TurboTax Easy Extension to e-file the extension request then you created an online account.  You have to sign onto the online account using the exact same User ID you used to create the online account - https://myturbotax.intuit.com/

If you cannot find your account -

Close all TurboTax windows on your web browser (including this one).  Copy and paste the account recovery website link onto a new web browser window and run the tool.

Use this TurboTax account recovery website to get a list of all the user ID's for an email address.  Run the tool against any email addresses you may have used - https://myturbotax.intuit.com/account-recovery

If none of the user ID's received will access your 2018 account, then use the option shown in blue on the account recovery website, "Try something else"

Or if you have signed onto your TurboTax online account and do not land on the web page with a Take me to my return button then follow this procedure - 

When you sign onto your online account and land on the Tax Home web page, scroll down and click on Add a state.  

This will take you back to the 2018 online tax return.