I'm happy to see you posting here in QuickBooks Community.
Yes, there's a way for your assistant to see only certain parts in QuickBooks Online. I suggest setting your assistant as a Limited Standard user and choose the Customer box. This way, the user can send out invoices for you without seeing all of the cashflows in QuickBooks.
Go to the Gear icon ⚙ and choose Manage users. If you can’t select this, you don’t have permission to manage other users. Contact an admin user for help.
Click the Add user button.
Select Standard user. More options will appear on the screen depending on the user type you select. Then, click Next.
Choose the Limited radio button. Then, checkmark the Customer box.
Choose user settings and click Next.
Enter your new user’s name and email address.
Once done, your new user will quickly get an email from firstname.lastname@example.org. It has your business’s name and a message that includes “Ready to get started? Let's go!”