micronucleus VS ThinkpadBattery

You can actually bit-bang low speed USB on Arduino [0]. There is even a bootloader [1] for ATtiny. It is obviously not compatible with Arduino, but those tiny boards look too cute to ignore [2]. Also, you can use USB port both for downloading firmware and your application.

[0] https://github.com/obdev/v-usb

[1] https://github.com/micronucleus/micronucleus

[2] https://cpldcpu.wordpress.com/2014/04/25/the-nanite-85/

The Digispark[1] and its clones use the ATTINY85 and offer flashing over USB via the Micronucleus bootloader[2] using V-USB[3]. It comes at the cost of a significant chunk of flash storage, but the convenience is excellent. Plus, V-USB support means you can emulate all sorts of USB devices. I've used it to build a DIY NES controller adapter, but it's commonly used as a cheap Rubber Ducky[4]. For projects where USB interfacing makes sense, it can be a good fit.

1: http://digistump.com/products/1

2: https://github.com/micronucleus/micronucleus

3: https://www.obdev.at/products/vusb/index.html

4: https://shop.hak5.org/products/usb-rubber-ducky-deluxe

So it looks like they're using the micronucleus bootloader for the linked example. That bootloader not only does the normal bootloader job, but also acts as a bitbanging USB device in order to communicate with the computer over USB. The USB bitbanging is a huge endeavor to undertake on top of writing a bootloader.

But do you guys know if I can reuse the case of original battery? If yes, I will only need the bms of this third parts battery. There is also some 3D-printing files for new cases :https://github.com/iam4722202468/ThinkpadBattery

It's very possible, I almost finished building a board based on the bq3060 charging IC but stopped because it could potentially be dangerous if I made a mistake and I'm not super knowledgeable in this field. Most of the work is done, someone who knows what they're doing could easily finish the design and get it manufactured https://github.com/iam4722202468/ThinkpadBattery/tree/master/KiCad/bq3060

See also the from-scratch Thinkpad battery with an ATTiny85: https://github.com/iam4722202468/ThinkpadBattery

I recently came across a project on github for a thinkpad battery. however, the image he provided made the battery look rather bulky. I was wondering if I could redo this project qwith the same hardware, except using a more streamlined case. does anyone know of a mockup/3d model of the thinkpad t420 battery?

Yes: https://github.com/iam4722202468/ThinkpadBattery