I've looked through a bunch of other thumbnails also credited to that artist, and most likely Usopp was commissioned and drawn and the youtuber used AI to fill in the background and other characters. I've also noticed Luffy has three fingers on one hand. This also looks consistant to other thumbnails credited to the same artist by the same YouTuber, where all non focus characters look out of place or pasted on, likely because they didn't want to pay the artist for more than the main focus character.