This tool (Google Analytics) is created for millions different businesses, and as we now that every business is different in their own way you can't expect it to be suited for your business, so you can just plug it in an wait for results. The beauty and the challenge is to configure it right to get meaningful data. Bounce is calculated on session level, session end in : 30 min (by default, can be configured), End of the day (time zone which is set in GA account), When Acquisition Campaign changes, end more end more. All of that affect the bounce rate. And much more things for example if your page is too long, have a lot of reading material maybe not possible to read in 30 minutes that is a bounce as well.
And avg. time on page is by default calculated (time difference between 2 hits)
My advice is that you need To create new marketing/tracking plan go briefly through your site see where you can add some events, goals etc. If the page is too long add a scroll tracking or timer trigger, or even combine them (example). I mean the beauty is that you can track whatever you imagine to track, but the point is to track what is relevant for your business.