@Andrew Katz new construction in a Class C neighborhood?

2. how to know where to find a B,C,D areas\neighborhoods in cities\ countries.3. how do I know if a property is worth the checking\ buying?

Figure out which best fits you and your personality and take it from there.  

Engage an investor-focused realtor/broker in your preferred market (this is not a residential realtor) and start 'tire kicking' the market and properties that fit your criteria.

I am now ready to hang drywall, but I'm not sure how to fit the already wired, electrical boxes through the drywall cutouts.

Investors who are active in gathering data on local trends while working with knowledgeable agents are able to pick neighbourhoods poised for growth and also comprehend how to best mitigate their risks as they prepare to maximize their return.Myth 3: Distressed Properties Are in Bad LocationsAnother misconception is that distressed properties only exist in bad neighborhoods.

Quote from @Deltrease Hart-Anderson: What are your thoughts on putting new construction in a B+/C- neighborhood?

to your point, if you save up so that you're in a strong financial position, house hack a solid duplex in a decent neighborhood, make smart decisions, and hold it indefinitely, you'll be fine.conversely, if you have $0 and somehow start out with a risky flip, or borrow $20K and spend it all on a magic mentoring program, or buy a terrible property in a neighborhood you've never been to, you may not be fine.

For C to B-Larger developments announced nearby-Increased activity in new construction-Building of commercial retail in or near the neighborhood. 

Sometimes even neighborhoods can vary street by street, so there's always a lot to learn!