When the mobiles were not having apps, we were still in the age of internet and websites, we could do all we do today on apps on websites then BUT we never had so many OS versions, we never had so many hardware configs (of phones today), our apps were running on browsers are they were not getting minimised of crashed when we got a call in between. the permutation and combination of fundamental mind map of Mobile Application Usability Testing is huge. but once you have it automation is the way to go. Appium, Android studio, Rest Assured (API), Components tests, integrations and on the fly smokes on device farms, the complex world of mobile test automation is eventually should be looked as a well organised matrix of all possibilities plugged in with right coverage and automation efforts, once you get the hang of it its not the nightmare you thought it would be.