I wanted to ask if the default Python 3 version on Ubuntu 18.04 LTS will always be on the Python 3.6.x branch, or might it update to 3.7 in the future?
If it will be updated to 3.7, is there a time frame for this?
Since 18.04 is an LTS release, it is unlikely that the Python version that comes with it will be updated to a new major version.
You can install Python 3.7 alongside Python 3.6, if you want, by running
sudo apt install python3.7 in a terminal, although, in my opinion, it would be better not to mess with multiple Python versions installed in your system through apt, since this could make your system unstable.
Instead, I am highly recommending to download and install the Anaconda Python distribution and use virtual environments managed by conda for the different Python versions you need.