ATtiny85. How do I burn a bootloader? What is the process? I want to use all six I/O pins of an ATtiny85, that means also using the reset pin as an I/O pin, however, I found out that if I disable the RESET pin, I can no longer program the ATtiny using an ISP programmer.

However, I found out that there are bootloaders for ATtiny which, as far as I understand, allow me to program the ATtiny through USB, without the need of the RESET pin.

My question is: How do I burn such a bootloader (such as micronucleus for example) to the ATtiny85? And how do I need to connect the ATtiny to USB after that (like this?)? How do I program it after that? Keep in mind that in terms of burning bootloaders and modifying fuses, I am a complete newbie.

Also, I'm using the ArduinoISP sketch as my ISP programmer since I don't have a real ISP programmer, but as far as I read, that won't be a problem.

EDIT: I do have this thing I bought for a couple dollars on eBay, which I don't know how and if I need to to use:
Yes, there are USB interfaces for ATtiny85. Just look at one of the open hardware designs out there, e.g. Digispark.
An USB interface is going to require two pins, so using one to save one pin doesn’t strike me as cost effective.
I agree with @FAD, that you don’t really need an USB stack, and simply could do a time-delay-at-boot thing with the UART pins.
HVSP (the high voltage programming) is not hard to do. If you’re already comfortable with ArduinoISP, you can try ScratchMonkey, which uses the same approach (with a 12V power supply and one transistor) to do HVSP programming.