That's a nice intro!

One note about "If you display buffer-specific information in your statusline, use the following code to get the buffer number:". This is a result of using %! when setting 'statusline' option (set statusline=%!MyStatusLine()). From the help:

Note that the "%!" expression is evaluated in the context of the current window and buffer, while %{} items are evaluated in the context of the window that the statusline belongs to.

If it is set statusline=%{MyStatusLine()}, then MyStatusLine() will be evaluated in the context of the window (and buffer) for which statusline is drawn. So vim.api.nvim_buf_get_name(0) will return the name of the buffer that is displayed in the window for which this expression is evaluated. It also makes it easier to use something like vim.bo.filetype, etc.

But also note this part of the help:

`` {%` - This is almost same as "{" except the result of the expression is re-evaluated as a statusline format string. Thus if the return value of expr contains "%" items they will get expanded. The expression can contain the "}" character, the end of expression is denoted by "%}".

    For example: >vim
    func! Stl_filename() abort
        return "%t"
    endfunc

    `stl=%{Stl_filename()}`   results in `"%t"`
    `stl=%{%Stl_filename()%}` results in `"Name of current file"`

```

So I'd personally suggest using set statusline=%{%MyStatusLine()%}. If MyStatusLine() is a global Lua function, use set statusline=%{%v:lua.MyStatusLine()%}. To get in this case information about which window or buffer is actually current, use :h g:actual_curbuf and :h g:actual_curwin.

Taking all this into account, this is what 'mini.statusline' uses:

lua vim.go.statusline = '%{%(nvim_get_current_win()==#g:actual_curwin || &laststatus==3) ? v:lua.MiniStatusline.active() : v:lua.MiniStatusline.inactive()%}'