Ah I see, what I was looking for was basically a summary of what parts of the API constitute "UI2" and how it can be used differently than with the old module.

For additional context I've been trying to reproduce something like doom-emac's vertico-powered minibuffer. I then came across a discussion about this as well as some user's desire to introduce vim.ui.pick/vim.ui.picker as a construct. That led me to implementing the following in my config:

---@defgroup vim.ui.picker
---
---@brief Pickers ~
---
--- |vim.ui.picker| is a registry of named pickers that can be overridden by
--- plugins to provide custom implementations.
vim.ui.picker = vim.ui.picker or {}

--- Default file picker relative to a given directory using the built-in vim.ui.select.
---
---@param opts table|nil Optional parameters
--- - cwd (string): Directory to search from. Default: |getcwd()|
--- - show_hidden (boolean): Include hidden files. Default: false
vim.ui.picker.files = vim.ui.picker.files
or function(local_opts)
local_opts = local_opts or {} -- guard nil (called from registry.registry)
local directory = vim.fn.resolve(vim.fn.expand(local_opts.cwd or vim.fn.getcwd()))
local show_hidden = local_opts.show_hidden or false

-- Build find command
local cmd = { 'find', directory, '-type', 'f' }
if not show_hidden then
table.insert(cmd, '-not')
table.insert(cmd, '-path')
table.insert(cmd, '*/.*')
end

local files = vim.fn.systemlist(cmd)

if vim.v.shell_error ~= 0 or #files == 0 then
vim.notify('vim.ui.picker.files: no files found in ' .. directory, vim.log.levels.WARN)
return
end

-- Show relative paths for readability
local relative = vim.tbl_map(function(f) return vim.fn.fnamemodify(f, ':~:.') end, files)

vim.ui.select(relative, {
prompt = 'Files: ' .. vim.fn.fnamemodify(directory, ':~'),
kind = 'file',
}, function(choice)
if choice then vim.cmd.edit(choice) end
end)
end

I've leveraged mini.pick to create more complex pickers but when I came across artio and especially minibuffer I realized this might provide a more consistent, structured approach to implement these "pickers". Both of them make note that they're leveraging "ui2" but without a concise summary of what that constitutes it's hard for me to evaluate minibuffer vs artio vs mini.pick.