Bundlepacks Add-on 1.21+

You can never have too many backpacks. They will help even in the most difficult situations, when you need to be sure to get that block of earth to your house in Minecraft. It's in those situations that you can pour a handful of earth into your bag with the Bundlepacks mod. This time, the backpacks are really cheater. You'll be able to store one bag in another bag, which means the number of inventory slots will be endless.

This is what a player with a backpack on his back looks like.

Simple Craft

How does it work?
To place an item, click the item on the bag or the bag on the item
Tap the bag or tap the bag on an empty slot to place the item
Tap the item in the air to drop all your stuff off.