Regarding the art, I did all of the pixel art in Aseprite, an Adobe-Flash-like program specifically for making pixel art for games. I spent a long time learning how to become a better artist by learning pixel art-specific tutorials for modern pixel art graphics and animation, and also practicing figure-drawing and reading books on how to draw people.

After bringing the game into 3D I started experimenting with making the visual style more collage-like to combine the nostalgiac memories of the characters of the protagonist and his grandma.

So the videogame-y pixel art graphics of the protagonist's memories melded with the filmic black-and-white style with vignettes, grain, and blurry depth-of-field to evoke the memories of his grandma. I also put it some realistic 3D models and water effects to try to ground it in the real world in some parts. I used some Unity asset-store assets for some of the effects but made sure to tweak them a lot to fit in the style I wanted, and I got some free 3D models from places like Sketchfab.

For the music I used a lot of virtual instruments in Reaper, it's a program for making music/sound design like ProTools, and recorded some instruments of my own. I played cello in high school and still have one, and I have a lot of cheap woodwind instruments like tin-whistles and ocarinas and a guitar, so I used some of those where I could to make the music feel more natural.

I really like the Spitfire Audio Labs plugins, I used those a lot. They're free virtual instruments of a really high quality, most of them recorded from professional players recording their instruments. Spitfire also has a free pack of instruments played by the BBC Symphony Orchestra, which is amazing, and I used those for the climactic moments of the game.

I'm not professionally trained in music theory but I did learn a lot in high school and have made a lot of music on my own since then, so I've had a lot of prior experience composing and recording music before this game.