ISP protocol is built in to the AVR chips as a way to upload programs. HVSP is another protocol built in to the chips. With ISP protocol, the reset pin is used to put the chip in to the ISP mode where the other pins (d0, d1, d2 iirc) are then used as an SPI connection to upload the program. LittleWire is an ISP programmer. AVR chips come by default configured with the reset pin as entering ISP mode.
HVSP mode works similarly, but the programmer needs to apply 12v to the reset pin. This means the reset pin can be used as IO after changing the settings in the chip to disable the reset pin functionality via any kind of programmer (even ISP). HVSP programmers are unpopular and cannot be made using a digispark alone because of that 12v requirement - you need some sort of 12v power supply and something to switch it - a transistor or mosfet or something like that. With a HVSP programmer you can upload programs regardless of the chip settings, or you can use it to enable the reset pin functionality again so you can then use an ISP programmer on it (this is what tiny fuse resetter circuits do)
A bootloader is a piece of software you can install on the chip which runs before any uploaded programs, which accepts program uploads in one way or another and uses the flash self programming features of the avr chip to install the program in to a different part of memory. Because a bootloader is software on the chip, it takes up some flash memory. USB is a fairly complex protocol so micronucleus (the usb bootloader) takes up about 2kb. You can get other bootloaders which accept uploads using different protocols, like serial or spi or even special single wire versions of serial. There are bootloaders for atmega which accept program uploads via light sensors from a computer screen (like a webpage or animated gif!) and others which accept upload via sound, by plugging it in to an audio jack on a computer or phone, so you can save the program as a wav and email it to someone you made a device for! Many of these other bootloaders are more compact than 2kb because their protocols can be made simpler than USB.
With a bootloader installed, you can disable the reset pin functionality if you feel okay about removing ISP programmer capability and just using the bootloader. This is how digispark works and how the chip is configured using the Burn Bootloader function. Notably the digispark burn bootloader function actually does not upload the bootloader, but a bootloader installer, which installs the bootloader when the chip runs after upload - this is because it can also be used to replace the bootloader on a chip using the already installed bootloader, but a bootloader cannot write over itself without breaking itself, so it happens in two steps. When making the installer program I have installed it on chips with ISP enabled and downloaded the resulting firmwares and verified by hand that the 'upgrade' installer does in fact install the bootloader to be exactly the same as if uploaded directly.
This installer is neat because it lets me make other bootloaders like the button bootloader where you connect reset to ground while plugging in the digispark and that makes it enter bootloader, but after that it can be used as IO by the program with no problems. Without the 'upgrade' installer people wouldn't be able to change the bootloader on their digispark without a HVSP/ISP as the reset pin is disabled so it can be used as a general IO pin and analog input.
I believe an attiny85 can be used to make a simple attiny fuse resetter, so if you have a few extra components like transistor, resistor, 12v power source like a 12v camera battery, you can use a digispark in theory to reset an avr or even another digispark so it can be programmed with an ISP. You can use an ISP to disable the ISP function and regain the pin's use as IO, as it does not take effect until the chip restarts after programming is finished.
