The Mac app hides rulers by default, but you can show them by selecting View > Canvas > Show Rulers or pressing Ctrl R.
Select View > Canvas > Show Pixel Grid on Zoom, or press Ctrl X, to see any edges of your layers that don’t align with the Pixel Grid once you zoom in past 600%. If you’re working on a design where it’s important to be able to see individual pixels, select View > Canvas > Show Pixels on Zoom or press Ctrl P to enable Pixel Zoom and view individual pixels when you zoom in past 100%. The Mac app measures layers in points - where one point is equal to one pixel on the Canvas - and when you export, you can scale things to different resolutions. It’s resolution independent and you can zoom infinitely to work at any level of detail. By default, you’ll view the Canvas in vector mode.
