I do a lot of frontend work on different apps and websites, but these are usually either an off-the-shelf product or something that somebody else has built. I have goals of making apps in the next few years but right now I'm focusing on learning programming better.
When I think about the closest thing I've built to a custom app, there were a few websites that I built using WordPress plus a really great plugin called Advanced Custom Fields. The ACF plugin let you define custom entries in the database and provided a very powerful backend editor…editor. You could add custom fields and editors and file uploads on the backend, and then display the data from these custom fields anywhere in the template on the frontend of the website.
I used this functionality to build a few 100% 'custom' designed CMS websites where we designed each page carefully, then replicated all of the content areas using Custom fields, we even added extra theme options they could toggle in the editing backend (things like color themes, alignment, choice of icons, etc). For each page on the live site there was an equivalent backend editor with exactly the same content.
Looking back on it, it worked but it really wasn't the Correct™ way to build a custom CMS. It was still vulnerable to 100% of WordPress vulnerabilities, but we weren't really using much of WordPress other than the backend editor and this plugin as a way to construct custom editors that a user could log into.
I think if I was going to approach the same projects now I'd be using something simpler, without any parts that we didn't need. I wonder if there are simple 'backend editor' builders we could use to generate the backend we need one time, instead of relying on something like WordPress as a software platform to run our 'backend editor builder' on top of.