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You can design pages, if you have the right permission set. Search for Permission Sets.
You need to have permission sets for Extension Management.
You can check the user permission sets attached to the User (Search Users > Edit the relevant User > Check User Permission Sets).
So typically Personalization is for end user and Designer is applicable for Super user or Administrators.
Apart from right permission set, you can only design when you are in Sandbox environment.
You can easily create a sandbox environment using the Sandbox Environment wizard.
The App Bar will display Environment name as “Sandbox” and you will be able to access Designer from the Settings.
When you are in Designer mode, you can do the design activities, such as:
Replace fields (or reposition them)
You do changes for different views (PC/Tablet/Mobile), etc.
The changes that you make will be saved as an Extension (and this is how it is different from Personalize) and Installed, which means that all the other users will also see these changes. You can also download the code to make it available in the visual studio code.