When are imports loaded?

[snippsat]

do they load to memory when the main thread starts, or when there is an actual call to the library?

Importing a module/package will always be fully imported into sys.modules mapping.
So the file/files with imported code will always be there even if you call it or not.
Eg:

# foo.py
def foo_func():
    return f'i am {foo_func.__name__}'

# bar.py
import foo

def bar_func():
    print(f'I am bar,hello also to <{foo.foo_func()}>')

if __name__ == '__main__':
    bar_func()

Output:
I am bar,hello also to <i am foo_func>

λ ptpython 
λ ptpython -i bar.py
>>> dir()
['__annotations__', '__builtins__', '__cached__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__', 'bar_func', 'foo', 're', 'run', 'sys']

# See that foo is also here,also the file foo.py 
>>> import sys

>>> sys.modules['foo']
<module 'foo' from 'E:\\div_code\\import_test\\foo.py'>

# So when call module foo in bar it's foo.py --> foo_func()
>>> sys.modules['foo'].foo_func()
'i am foo_func'