How could I make a spawn platform for wandering traders in order to get Nautilus shells?

An AFK fish farm is a dead easy way to get Nautilus shells. Just AFK overnight and you'll get loads of them. Before you do, make sure you have enough storage space for the items to automatically feed into!

Follow Xisumavoids tutorial for a visual walkthrough:

Well there might be a easy way to get them from drowned. First try to illuminate the waters nearby so there are going to be no drowned automatically spawning. I recommend glow stone,or if you can’t, use torches and glass. Secondly, build a small hut which's interior is pitch black. The drowned will spawn there, and you can put a villager in another small hut underwater so the drowned are attracted to it. Then put cactus in front of the hut’s only entrance, so the drowned will receive damage from the cacti. Items float in water, so all you need is to afk above water so the items automatically float towards you. Hope this might help.