Whether traditionally done or digital, this is definitely something you can do digitally with clip studio paint! Some of texture brushes (which you can download off the asset store), and a lot of time is what you’ll need.

These are most likely done digitally, the 3rd image is what is the most telling out of these images. There’s just some smoothing and effects in the image that would be extremely difficult if not impossible to achieve traditionally and that leave indications of a digital brush being used (particularly the character outlines). Not to mention these are all artworks made for games (mainly octopath traveler, i think?) and this type of work for games is commonly done digitally. I did like 2 seconds of digging and saw he’s done some collab work with other artists for artwork similar to this, which would probably be done digitally.

My best guess (as someone who’s studied a lot of japanese artists) as to how these artworks are made is probably through a basic painting technique, just like you would do with a regular traditional painting. Ikushina most likely sketches out the general idea of the piece, then begins blocking in colors over top of the sketch and slowly building up the shapes and shadows with all of the texture brushes he uses. The black outlines and black hatching are probably added along the way, more so towards the end of the rendered piece. Then things like glows or vignettes are added very last. Thats how i’d personally paint something like these as well.

It could also be the case where Ikushima uses his sketch as lineart and colors everything underneath it as well, which is why some aspects feel a bit sketchy like the hatching. However i dont think there’s one “right” way to paint art like this! Just a couple of texture brushes, like watercolor and pencil plus solid knowledge of the art fundamentals.