I've gotten a few people asking if I'm using markers or digital because at times it could look like either. @R-L-Z asked how do I actually do it and that took a turn into a whole post.
So quick answer it that it is digital.
To start off I acknowledge that I'm not someone who should be giving Illustration advice. I am a web developer and commercial artist for my day job. So I "professional" use all these tools and get paid a living wage for it but I don't make a living from drawing what I post here. The key thing to note is that, in my day job, I often need to make digital images that look old or hand made. I bring about a fourth of that in my personal work here but the other three fourth is painfully time consuming. That is why, at times, it does look like I'm drawing in marker. I'm using the same tools that I would use to make a digital image look like it was physical media.
Before I explain the process I need to explain two bits of advice from Fiona Staples (the artist of the comic Saga) and Darwin Cook that explains the logic I've been using in process.
Cook knew for a while he had terminal cancer and would give advice that was effectively figure out an style that gets the work done and doesn't waste time. Cook was one of the best modern illustrators and you can see how his style follows a similar journey as Moebius. Both spent the later years of their career figuring out how reduce the amount of marks they make in their art. Granted in entirely different ways. Cook's Parker books being the most notably of his efforts I can think of.
Staples gets asked why she doesn't ink her art and her answer is basically "I've already drawn it once. Why would I want to draw it again." More to the point her style doesn't use the classic pencils, inks, and color process the vast majority of comic have historically use. She maybe has thumbnails but she leverages her digital tools and does everything at once. She gets her art done quickly.
Basically I don't get a lot of time to do all this stuff so I need to work quickly if I want to get anything completed.
So here is the thing, while I have a lot of custom digital brushes and textures from work, I'm just using Clip Studio Paint with a stock brush and a stock eraser. I'm using them in particular ways but it isn't anything extra. The brush I use is the "Opaque Watercolor" brush which is the mid range version of the CPS watercolor brushes (transparent and dense being the lighter and heaver ones). Despite being called "opaque" it has some pressure transparency. The easer is the "Kneaded Eraser" which I use to "shave" lines thinner or work the line art. Any line with a fuzzy edge was one I shaved thinner with that eraser. Both these tools have more specific use in making images look like they were made using physical media like markers but I'm just half assing it to save a lot of time.
The actual process is just old school comic penciling but instead of using blue and red pencil I'm using weird colors. I draw the figure in one color and start adding clothing and objects in other colors. I'll draw clothing on a different layers which means it doesn't blend with the colors on other layers. That produces abrupt color change instead of gradual color blending. Combine that with the very old layer setting of "lock transparent pixels" to recolor lines if needed and that is basically the process. It is just 20 year old Photoshop techniques being used in modern day Clip Studio Paint.
The only other thing to add is that I work from a color pallet that I created from sampling colored printer paper and Post-It notes. That part is kind of weird but it is mostly because I suck at color theory and I think it is funny to use that as a starting point.
Nothing super special other picking what I want to put my effort into to get thing done quickly. Like most art styles it will probably only be successful depends on the "content" of the work to be successful.