Using the classic interface, it takes a 4 clicks to open the solution and entity customization window:
Settings->Customizations->Customize the System->”Experience the new way to customize the system”
Navigating using the new UI menu is even worse at 6 clicks. This might not sound like a lot, but the time spent waiting for windows to load adds up. Thankfully, accessing the solution customization area has never been easier. Please note that you must be a Dynamics 365 administrator or customizer to edit the solution.