NorCal.Social can be access via the web both on the desktop and on mobile devices. The website is “responsive” and works quite well on phones – I personally prefer it over using an app! But if you prefer an app on your phone – many people do – head over to joinmastodon.org/apps for a list of available apps you can install. At the top are the official Mastodon apps and if you scroll down you’ll see a large list of third party apps. There are a lot of options and you might want to try a few different ones to find the what’s best for you.