how do national spirits work as in adding one/remove one/modifying one/IDs

"National spirit" is just another way of referring to an idea which is set to the type "Country". You add/remove/modify them in your using the Ideas tool.

To add a national spirit to a country via a focus tree, for example, you would use the code:

add_ideas = IDEA_ID

And to remove it:

remove_ideas = IDEA_ID

If you want your nation to start with a national spirit, you would need to add it to the country's history file (if doing by code), or by adding it in the Misc. tab in the Country tool (if doing it on this site).

The idea ID will follow the structure:

[PREFIX_]idea_name

The prefix is optional. If your idea list doesn't have one, ignore that bit. The prefix is case-sensitive, and cannot start with a number. E.g. "ExAmPLe" is a valid prefix, and will be used as such in the idea ID.

The idea name will be converted to lowercase, will have any non-alphanumeric characters removed, and spaces converted to _. E.g. "It's Time!" becomes "its_time".

So if we take the two examples given, the idea ID is "ExAmPLe_its_time"