I think a bit of context about the ultimate goal here would help - what you are asking sounds bizzare as the Python code for QGTS is far more dependent on the QGIS version than the Python version (apart from Pyhon 2 vs 3, which in any case was a major change in QGIS.

To answer the question "will it make any difference if I run code written in Python 3.6 in QGIS 3.4 (Python 3.7)" it depends on the script, but Python tends not to make radical changes on point releases so you may be OK.

The other question is a no - QGIS installs its own version of python with all the QGIS binding libraries, and each Python version is self-contained. So no, you could not call qgis functions from your 3.6 installation unless you compile qgis yourself to use Python 3.6, as far as I know.

I think a bit of context about the ultimate goal here would help - what you are asking sounds bizzare as the Python code for QGIS is far more dependent on the QGIS version than the Python version (apart from Pyhon 2 vs 3, which in any case was a major change in QGIS.

To answer the question "will it make any difference if I run code written in Python 3.6 in QGIS 3.4 (Python 3.7)" it depends on the script, but Python tends not to make radical changes on point releases so you may be OK.

The other question is a no - QGIS installs its own version of python with all the QGIS binding libraries, and each Python version is self-contained. So no, you could not call qgis functions from your 3.6 installation unless you compile qgis yourself to use Python 3.6, as far as I know.