I'd like to configure Vim 8.2 on Ubuntu 18 so that when I open code in different programming languages, different settings load. Eg. indentation, color-theme, plugins etc.

I found ftplugin to be the recommended way to achieve this setup. Let's assume I want to setup VIM for python and markdown. I've installed the plugins (NERDTree, ale etc.) using VIM's native package manager. I've put some plugins like NERDTree in the /start folder while some like ALE in the /opt folder. This is because I want NERDTree to always load when I open any file using VIM while I want to conditionally load ALE when I only open a python file (and not markdown).

From what I've read in other answers and blogposts, we need to do the following -

1. Add filetype plugin indent on at the start of the main ~/.vimrc.
2. Create folders ~/.vim/ftplugin/python.vim and ~/.vim/ftplugin/markdown.vim.

"file => ~/.vim/ftplugin/python.vim
setlocal number
setlocal autoindent
setlocal shiftwidth=4
packadd! ale

"file => ~/.vim/ftplugin/markdown.vim
setlocal number
setlocal autoindent
setlocal shiftwidth=2

What will be the difference between adding ftplugin in the /after directory ie. ~/.vim/after/ftplugin/markdown.vim as opposed to ~/.vim/ftplugin/markdown.vim?