Currently it seems that brave playlist adds the quality at the default 360p, at least on my iPhone 11 and iPad - perhaps if one manually selects 1080p, then “add to playlist” will add it at 1080p, I have not tried - it would be advantageous if “add to playlist” saved the best quality that makes sense for the device’s display.
Higher quality saving is great because one cannot change the quality afterwards, and higher quality would mean that I can actually read the text on slideshows in presentations - which is my main use case of YouTube.
For implementation details, if youtube-dl is being used behind the scenes, it provides quality controls. If brave browser is just fetching whatever quality that youtube served it, then perhaps some magic can occur where it selects the highest quality option in the youtube video player, before it saves to playlist.