All fields in InDesign can be entered in any measurement system. So, if you want to make an 8"x10" document and go to File>New Document and the window looks like this:
…you can type either 8 in or 8" into the Width field like this:
…and when you move to the next field, it will convert the eight inches into the equivalent number in the unit of measure that it is using at the moment, like this:
… (203.2 milimeters is the same as 8 inches). You can also enter a different measurement unit into other fields once the file is created, such as the width or height of a frame, or the position of an object in the X and Y coordinates.
If you would rather just work in inches instead of having to type the inches mark or abbreviation, go to InDesign>Preferences>General and select the Units & Increments tab. There you will see Ruler Units for Horizontal and Vertical at the top of the window. Set them to Inches and all of the fields will display in inches. If you do that to an open document, you will change the unit of measure for that document. If you do it while no documents are open, it will change the unit of measure for any new documents you create. To change the unit of measure for existing documents, you will have to open each and make the change.
I'm happy to have helped.