Explanation
Graphical user interface, text, application Description automatically generated

Box 1: List box
A List Box control always shows all available choices (unlike a Drop down control) and in which the user can choose more than one item at a time (unlike a Radio control).
Box 2: Combo box
A Combo box control allows you to search for items you will select. The search is performed server-side on the SearchField property so performance is not affected by large data sources.
Single or multi-select mode is configured via the SelectMultiple property.
When searching for items to select, for each item you can choose to show a single data value, two values, or a picture and two values (Person) by modifying the Layout settings in the Data pane.
Reference:
https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/controls/control-list-box
https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/controls/control-combo-box