Is this a collection of existing plugins, or new ones?

[MATOO-Dev]

For example, is snacks.lazygit the same as kdheepak/lazygit.nvim, snacks.notifier the same as folke/noice.nvim, and so on?

Or another way to put it: should I install one over the other, or both? What are the differences?

[folke]

of course they're not the same. Check the docs