This looks amazing! Thank you!

Also, is there a tutorial on how to make a basement like this? Does it require a mod or anything?

Thank you sooo much for commenting I'm glad you like it!

I'm sure there are tutorials somewhere... but, here's a quick "how I do it":
1 - the house must have a foundation in order to have any kind of basement. Then delete the foundation except for 1T around the outside edge.
(I linked in the Post to Moo's Grid Adjuster, if you want to make 1-click or 2-click - actually, any-other-than-4-click - foundations)
2 - use any modular stairs - and dig down into the ground under the foundation with the stairs 3 times. (So it goes 3 stair clicks deep)
3 - use the flatten terrain tool to lower all the ground to the lowest height of the stairs.
4 - add your walls, etc. Done.

Each stair will dig 4-clicks. Your standard in-game foundation is 4-clicks. A livable level for the sims is 16 clicks. Using those specs, you can adjust accordingly if you want to use a shortened foundation. Like, this house has a 2-click foundation, so the basement is 14 clicks deep. (2+14=16 clicks total) So, the basement is 3 stair clicks (12 clicks) + another 2 clicks with the lower terrain tool.

I hope I didn't confuse you more
Play around with it and it will make sense. If you do find a tutorial, make sure it is a newer one! There are a few out there from looong ago that are really confusing with a lot of unnecessary steps involving floor tiles etc *shudder* Good luck!