There is no official bootloader for the t85 - it is not an official board, and there is no "official" bootloader.
The most common t85 bootloader is the vusb/digispark/digistump one, which requires a few external components, connects directly to USB, and requires the digispark core (my core has more and better features), and eats a whopping 2k of flash (1/4th of the flash on the t85 - leaving only 6k for your sketch; the serial bootloader my core includes only takes 640b).
I would not be surprised if chips were available pre-loaded with the digispark bootloader - though be aware if you are thinking of getting such pre-bootloaded parts, they sometimes disable reset on them to get an extra I/O pin - if that has been done, you cannot reprogram them with ISP, only via the bootloader. HVSP programming is required to undo that.
Honestly, for the t85 (and other parts with 8k of flash or less without a hardware UART), I recommend not using a bootloader at all, and just programming them via ISP. Bootloaders on parts without hardware UART are always kind of janky, as are bootloaders on parts without hardware bootloader support (basically all classic-avr* attiny parts with a couple of exceptions). You get more flash, and you don't have to worry about what the bootloader might be doing on startup messing up your setup. I only use bootloader on the 841, 1634, and 828 out of the classic attiny - the rest I just program via ISP.
I'm not sure off the top of my head if the STM boards can be used as ISP - I don't see anything immediately obvious in the arduinoisp sketch that would cause a problem, but I never hear of people doing it on these forums. People almost always use a USBAsp, USBTinyISP (both are under $5 shipped, get the 10p-6p adapter for the USBAsp if you go this route), or a classic AVR based arduino like the Uno/Nano/ProMini running the arduinoisp sketch (nano and pro mini clones are also under $5). I usually use USBAsps, though I have a nano with the ISP wires an 10uF cap soldered onto it (wires glued to the back with hot-glue - this keeps them from breaking off during handling, without it they break very easily right where the solder meets the wire) as a dedicated Arduino as ISP programmer.
- as opposed to megaavr, like the ones my megatinycore supports - ones where the part number ends in a 0 or 1 followed by a 2, 4, 6, or 7 - these all have a hardware uart and bootloader support.