Backend admin edit post customization without plugin

Hi All,
My project is to create a WordPress website with my own theme and with no external plugin (or at least a minimum of them) to have a better understanding (and learn) how wordpress works and to be able to adapt my future devs.

I already created the home page of the front-end with the menu I wanted (with the theme) and a simple YouTube plugin. (I used bootstrap for the positioning and sass to generate the css)

One of my last difficulty is on the admin back-end custom post edit (without external plugin) to improve the customer post edit experience ( I want to keep it in the admin menu at the moment).
I do not understand how I can fully customize the admin back-end edit custom posts .

I managed to :
– create the custom post type
– Modify the column list of the custom post list
– Create the taxonomies
(at the moment I do not use metabox because I do not need it).

But I do not know how to modify the appearance of the admin edit custom post :
+ to aggregate many taxonomies on on display box
+ to add css to the edit post
+ add admin boxes
+ Change my taxonomies as dropdown or a list box
+ Change my taxonomies as checkbox without leave the possibility to add other values
+ …

I someone could help me on the methodology (and functions that I have to use) or tutorials to follow it would be a great help.

Thanks