What makes setup() break plugins installed outside of "lazy"?

[JohnFinn]

Why does adding this line at the end causes the existing code BEFORE to stop working?

-- many calls like require("pluginFoo").setup(...)

require("lazy").setup()

sample error messages

Error executing vim.schedule lua callback: ...eovimPackages/start/gitsigns.nvim/lua/gitsigns/async.lua:85: The coroutine failed with this message: ...ck/myNeovimPackages/start/gitsigns.nvim/
lua/gitsigns.lua:135: module 'gitsigns.attach' not found:
        no field package.preload['gitsigns.attach']
cache_loader: module gitsigns.attach not found
cache_loader_lib: module gitsigns.attach not found
        no file '/nix/store/k85k151qrrl98gfbj5z00h36a5kj4fwy-luajit-2.1.1693350652-env/share/lua/5.1/gitsigns/attach.lua'
        no file '/nix/store/k85k151qrrl98gfbj5z00h36a5kj4fwy-luajit-2.1.1693350652-env/share/lua/5.1/gitsigns/attach/init.lua'
        no file '/nix/store/k85k151qrrl98gfbj5z00h36a5kj4fwy-luajit-2.1.1693350652-env/lib/lua/5.1/gitsigns/attach.so'
        no file '/nix/store/k85k151qrrl98gfbj5z00h36a5kj4fwy-luajit-2.1.1693350652-env/lib/lua/5.1/gitsigns.so'
stack traceback:
        [C]: in function 'require'
        ...ck/myNeovimPackages/start/gitsigns.nvim/lua/gitsigns.lua:135: in function 'setup_attach'
        ...ck/myNeovimPackages/start/gitsigns.nvim/lua/gitsigns.lua:191: in function <...ck/myNeovimPackages/start/gitsigns.nvim/lua/gitsigns.lua:166>
stack traceback:
        [C]: in function 'error'
        ...eovimPackages/start/gitsigns.nvim/lua/gitsigns/async.lua:85: in function 'cb'
        ...eovimPackages/start/gitsigns.nvim/lua/gitsigns/async.lua:127: in function <...eovimPackages/start/gitsigns.nvim/lua/gitsigns/async.lua:126>
...yNeovimPackages/start/noice.nvim/lua/noice/util/lazy.lua:6: module 'noice.config.views' not found:
^Ino field package.preload['noice.config.views']
cache_loader: module noice.config.views not found
cache_loader_lib: module noice.config.views not found
^Ino file '/nix/store/k85k151qrrl98gfbj5z00h36a5kj4fwy-luajit-2.1.1693350652-env/share/lua/5.1/noice/config/views.lua'
^Ino file '/nix/store/k85k151qrrl98gfbj5z00h36a5kj4fwy-luajit-2.1.1693350652-env/share/lua/5.1/noice/config/views/init.lua'
^Ino file '/nix/store/k85k151qrrl98gfbj5z00h36a5kj4fwy-luajit-2.1.1693350652-env/lib/lua/5.1/noice/config/views.so'
^Ino file '/nix/store/k85k151qrrl98gfbj5z00h36a5kj4fwy-luajit-2.1.1693350652-env/lib/lua/5.1/noice.so'

A bit of context

I have a working neovim setup with nixos, and I'm trying to migrate to lazy.nvim.
My plan was to move each plugin I have one-by-one to use lazy, so I can test each step.
Since my code was always working I didn't expect new statement at the end of file to break it. I'm new to neovim and this blows my mind, as I can't find an explanation to this behavior.

Could somebody please explain how the last statement could break all of the seemingly unaffected logic?

[iton0]

Are you using either of the two ways to setup your plugins? If you are setting up lazy and your plugins in the same file you need to setup your plugins within lazy setup as shown in the link. I hope this help.

[JohnFinn]

My question is rather about how does "lazy" work under the hood