Wow. Amazing job with this. I think this is my favorite game jam game I've played. Bunch of thoughts:

What I Liked:

Very impressive all you were able to build given the short amount of time.

The main menu was great - the background both provides something visually interesting AND serves as a tutorial.

The music was solid and sound effects effectively juiced the right points. Other little bits of juice that I noticed and thought were effective was having the item icons shake in the top left when introducing new types.

I think the tutorial was really well done. Demo'ing the placement of items with just a bit of overlaying text (that you don't even really need) and then immediately putting the player in a situation to try it out. Also, introducing each new item with a fitting level was done very well.

Overall, I thought the level design was great. Admittedly, I haven't gotten through the whole game (yet) but I think there was a nice challenge curve.

I also think in general, the concept is very clever. At the surface, it seems very simplistic, but as I went through the levels, I started to realize the possibilities, choices the players can make, and interesting situations you can create (and could easily expand on with more items).

But I think my favorite part of the game is the humor - found myself actually laughing out loud from the commentary.

Critical Feedback:

The game might possibly too challenging. I would be interested to hear how others feel, but I got kinda stuck around level 5 or so (not quite certain which level).

I'm wondering if having Easy, Medium, Hard modes that simply adjust Billy's speed might be a nice way to address this? Then you can cater to a variety of audiences easily.

Also, a level selector would be nice. I know you might not want to open up later levels until the player beats them for the challenge, but I don't want to have to beat all the early levels again to get back to where I was stuck haha. Maybe having  "level passwords" for each level that you get after beating it would be a way to get around this? (I'm imagining how old games used to do this, like Megaman X https://strategywiki.org/wiki/Mega_Man_X/Passwords)

Nit: I tried pressing the icon in the top left at first to place the springs (because it looked like that was how you did it from the tutorial and I wasn't super clear from the text). Didn't take long to figure out, but I think just making the text say "Tap WHERE you want to place an item..." (and really cool to show a tap, with a little bubble icon or something)

Questions:

Ahhh a Phaser developer! I used it too a few years ago (I think it was Phaser 2 then), how do you like it?

game is made entirely on a phone

^ So you actually wrote the code, GarageBand, and Assembly using your phone?

Really enjoyed playing this game and I feel like I learned some game development tips by just playing it. Well done!