How do I add all the files in my Projectile project folder to Cape completions and/or dabbrev?

[nerdictranslator]

Hey there,

So I've been experimenting (for some time, really) with the possibility of replacing Obsidian with Emacs as my primary writing environment. I'm taking it slow, trying to recreate all the functionality that is even half-way essential to me. Obsidian has this useful little plugin that'll read all the files in my vault, and make the words available for auto-completion (especially useful for weird fantasy names). It's similar to what VS Codium does with (my very simple web) projects too.

Now I have Cape/Corfu up and running. I know that dabbrev can read all of my open buffers for autocompletion candidates, and I've even experimented with a script that compiles everything in my markdown files into one single file, stripping out the duplicate words, and loading that into an Emacs buffer to sort of build my own word database as I go... but I'd really like a more convenient way to do this.

Is there any way to make Cape read all of my project files in the background and use them for auto-completion?

Please note that I am still something of an Emacs/coding beginner, so you might need to explain some things like I'm 5.