Usually with a rabbit of unknown origins, it's considered a mixed breed because there's no way to determine any particular breed. Unless there are pretty distinct breed traits. Like your rabbit has. There aren't many long coated lop ear breeds, so that narrows it down significantly and can help pinpoint a particular breed.
If you're in the US, American fuzzy lop would be the most likely breed, but your rabbit doesn't match the breed traits precisely. The closest match I can see is to the Teddywidder breed. Though I'm no expert on breeds, and this is just my best guess from what I do know.
https://rabbitpros.com/teddywidder-rabbit-breed/
Understand, this would be fairly unusual considering how extremely rare of a breed it is, especially if you are located in the US. If you're in Europe, I think it's still a fairly rare breed there as well, unless you're in Germany where the breed originates. So anywhere else but Germany, it would be a less common breed, but though it would be unusual to come across a rabbit like this in a shelter, it's obviously not completely impossible.
I presume the shelter would have gone over this with you, but in case they didn't, I figure I should bring up the importance of regular grooming with long coated breeds. So regular brushing or blowing out the coat, and trimming when necessary (particularly in summer if your rabbit will be kept anywhere that could result in overheating), will help prevent that fur getting matted up. Which can happen fairly quickly if not kept right on top of with the grooming.
You've got a good start with a fresh trim, now just to keep up the grooming as it starts growing out. If you do opt for regular trims, if not going to your experienced rabbit vet for it, just be aware that rabbit skin can be very delicate, and so the proper precautions need to be taken to prevent damaging the skin. Even when just brushing with too harsh of a comb, it can rip the skin if not being very careful. If in doubt, just consult with your vet, if an experienced rabbit vet.
https://wabbitwiki.com/wiki/Angora_and_other_long-haired_rabbits
https://wabbitwiki.com/wiki/Hot_weather_concerns
Though a long coated breed can be more of a challenge, it is doable and just takes practice getting the whole grooming thing down. I'm not sure what other active members here have a long coated breed, but feel free to post any questions you have, and hopefully someone will be able to help with the answer.
Your bun is absolutely adorable
Welcome to the forum!