In somewhat reverse order, Digital to Analog Converter usually refers to an adapter that lets you plug in headphones through the USB port. Apple does a better than average job for audio things except when it comes to actually having ports to plug audio equipment into. Dongle is pretty cheap but I like the Qudelix 5K. While they don't pay me to shill it would be rad if they did.

While Apple Music doesn't have everything in their library... I don't think any of them do... the lossless support is really nice! Windows environment sucks but moving out of it is a good way to have less losses in your signal chain. Still not really sure of a good Windows service to cache and stream losslessly though that matches the apple library.

For the good ol' m8 Sennies, I'm seeing 300ohm impedance which can limit how loud they can get. I think this community is getting better at trying to not blast out their ears with open backs so you might not even need an amp at all! Yeah, the amp helps if you want to get L O U D. Thing is, it's hard to find papers that explicitly state how different listening when it comes to having more powerful amplifiers. Until some more research comes out on it, we're kind of in a spot where people are going to defend spending lots of money. You'll probably do fine with the dongle if what I see from https://old.reddit.com/r/headphones/comments/ecf1ud/apple_usbc_to_35mm_adapter_impressions/ and https://www.headphonesty.com/headphone-power-calculator/ are right, but the build quality on the dongles are iffy so I would recommend getting two apple dongles when the first one breaks in a few months like mine did, a 5K, or something more expensive if you're curious.