[misc]: code optimisations of typing class, typos and grammar

[wthrajat]

This PR fixes:

• Now using shorthand notation of typing.Union method, as recommended in the official documentation here: https://docs.python.org/3/library/typing.html#typing.Union
  
  This is a mini optimisation as we don't have to import the typing module now.
• Comments, typos and grammar improvement.

NOTE: Nothing to test or screen-record before and after as no functionality has been changed.

PR Checklist:

• All new features have been tested (nothing to test)
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct
• All new features have been documented (nothing to document)

[MuhammadMuradG]

Good typos catching. However, I prefer split this PR to 2 PRs to address each of the mentioned problems separately (one PR for typos and one PR for Union class). We can bypass this splitting problem in this PR and adopting splitting strategy of different tasks in the future PRs.

Changed in version 3.10: Unions can now be written as X | Y. See union type expressions.

Regarding to the adopting | instead of Union in typing module, I think this will limit the Python versions supported by our application to 3.10+ instead of 3.8+. Do we really need this?