Keeping up with IDEs and Virtual Environments...

[snippsat]

I think I am missing a piece about virtual environments: we don't use pip to add a Python interpreter to the virtual environment. So how do we connect a specific Python interpreter version to a virtual environment? What are the commands?

You are talking about venv that comes with Python?
Usually use the lasted version have on OS,but can use any version that is installed.
Example:

G:\div_code
λ python -V
Python 3.10.5

G:\div_code
λ python -m venv new_env

G:\div_code
λ cd new_env\

G:\div_code\new_env
λ G:\div_code\new_env\Scripts\activate

(new_env) G:\div_code\new_env
λ python -V
Python 3.10.5 

Let say i want to use Python 3.7 that have installed,then can use py(comes with Python).

G:\div_code
λ py -3.7 -V
Python 3.7.3

G:\div_code
λ py -3.7 -m venv new_env

G:\div_code
λ cd new_env\

G:\div_code\new_env
λ G:\div_code\new_env\Scripts\activate

# Environment will now use 3.7
(new_env) G:\div_code\new_env
λ python -V
Python 3.7.3

There are 3-party packages that can install a new Python version to a environment(not on OS) like Anacondas conda ,
or Poetry with pyenv on Linux.
Poetry is really good so worth to take a look.

Ok on IDEs non coming with an interpreter. I guess the IDLE did when I downloaded it.

No,but PyCharm will usually find any Python version if there is any on the OS.
Look at Configure a Python interpreter PyCharm
There see how to choice an exiting interpreter or virtual environment.