I have some very specification questions concerning segmentation

I have some very specification questions concerning segmentation. First question is concerning everyone's experience with segmenting by product purchased, or by customer type.  What type of segmentation leads to more conversions statistically speaking? Next if I elect to set this up by product purchased am I able to setup this using multiple items purchased? Example you own a bakery you have customers who purchase blueberry muffins, and also purchase dark roast coffee.  Next when I click on create segment I'm noticing the segments are pulling from tags which leads me to my next questions. Does this mean I have to tag each customer in shopify with Blueberry muffin, and dark roast coffee? Finally how do I automate these segments for future orders so the list grows i.e. someone buys a blueberry muffin tomorrow. Thank you all in advance for your expertise please do not try and sell me on your service.  Sorry had to throw that in but this happens often to me and is annoying.