Hi
Most Xerte projects (depending on how they were authored and what was added) should work on mobile devices via a direct link but I guess what you mean is can they be downloaded for offline access? Here's a very quick answer to that:
You can use the Offline Export option with XOT projects to create a zip that can be downloaded and extracted on mobile devices and should work in that way. However each Mobile device and OS works differently and even each the version of the device or OS can vary. The basic workflow is to download the zip to the device and extract that zip so that you can access the index.html file within the extracted folder. On different devices you will probably need different apps for this to work so it's a question of trial and error and then documentation for your learners. It can be tricky because every learner may have a different device/OS and even then you have to have deliberately authored without any content that requires web access.
Bootstrap projects do not currently have Offline export and none of the other export formats in any of the projects will work offline in this way.
So it is possible and a few projects I've worked with directly do this specifically because they need offline access on mobile devices where connectivity isn't possible but you'll have to work out the best workflow and apps that work for your students and their devices and you'll have to author with offline access in mind or explain that online content will obviously not work without connectivity.
HTH
Ron