Graham bemethavinci The site is built with a template but not actual bootstrap
it belongs to a longtime xtgem user but is not hosted on xtgem (although some of the code and css is on xtgem)
The web version seems to be this http://www.free-css.com/free-css-templates/page127/coolblue-v1.0
but it switches to a totally different css/html template for mobiles (not 'bootstrap' either)

There are hundreds of sites offering many thousands of templates
They all do the same basic thing some better than others
most rely on checking screensize some may do some user agent sniffing to deliver mobile content
The rest is just layout

Some browsers cause problems even on devices that can use the markup in their native browsers
notably most operamini and ucweb versions do not like any form of 'dynamic' content

xtgem actually has a built in system for mobile content when you are using xtgem_template.css (or if the file exists and has at least 1 class in it)
if the device is detected as 'web' then the xt_touch class is added to the body tag and this can be used to add extra css by reference to this class
This is the system used by the built in css templates
The blogs twig 'data' array also has a device detection feature in the data.device value which returns web/touch/mobile
xtgems xt function <xt:get_device_template /> used to return web/touch/mobile but these days just returns web/mobile in most cases
The google tool to check if a site is mobile friendly is https://search.google.com/search-console/mobile-friendly
Although you may have issues using the tool from a mobile (typical google)