Layer Blend Modes are so so so important to working in digital art. There's a ton of math that goes into figuring out how the layers should blend together, which is why some of the modes you can pick are literally called Multiply, Add, Divide, and Difference (that's subtraction). The graphics software takes the color values of your base and blend layers and runs a calculation to get your resulting layer appearance. The ones that don't have specifically mathematical sounding names are still doing calculations, but they're more complicated (think linear Algebra and higher). Some of them, like dodge and burn, are named for actual photo editing techniques.
While it's not super important to know about the mathematical side of blend modes, I think it's worth knowing at least enough about how each of the categories of blend modes works and why they do what they do; if for no other reason than having a starting point when you start experimenting with them in your work.
An overview of the basic blend modes and how they work from Genevieve's Design Studio: Accessible with minimal color knowledge; practical and illustration focused. https://youtu.be/kMc87hQrJd0?si=TWCB365pKSfWS8p0. (16 minutes) This creator also has a ton of free resources you can download, including a Blend Modes cheatsheet, but fair warning: you have to create an account to get them!
Want to learn even more about the math-y stuff? It has great film visuals! A video from FilmmakerIQ: You need some basic knowledge of RGB color models, understanding of values/luma, and at least a tenuous understanding of Algebraic formulas. (26 minutes) https://youtu.be/F7_kaTP7_W4?si=x0urqXZ8f51nQVKl
