Hi @bbsin,
Could you please clarify more clearly that you have a Form rather than a Gallery?
In addition, could you please check our official doc to learn more about the form generated based on the SharePoint list, then you should know that a Combo Box will be automatically generated based on a Choice or Person.
Why you change your column type, I could not afford to provide different solutions every time you changed your column type.

Please do keep your Text type for Designation and Department column, and keep the Person type for your person name column. And then create a form based on the SP list, you could get a form with a Combo Box for the Person Name and two TextInputs for the Designation and Department.
Just set the Default property pf the TextInput as below:
ComboBox1.Selected.JobTitle
ComboBox1.Selected.Department
Note that Combo Box represents the control within your form, replace it with yours.