Hi there!

I always found that the estimation questions that I faced in interviews were intentionally more abstract than the formulaic "How many _____ are bought in ______ each year?" problems you tend to find online. For that reason I think the best practice is to categorise the different techniques and ways of approaching estimation questions rather than categorising the questions themselves.

I would look to approach these questions in a number of ways in my head before deciding on the easiest method (or the one that involves the least blind guesses).

1. Consider Demand: This is often the most simple way to break down an estimation problem involving the quantity of products/services sold in a country. If asked how many men's haircuts are sold each week in the USA, for example, you could assess how frequently you require a haircut and extrapolate this to the population of the USA.
2. Consider Supply: Conversely, you may want to approach this from the hairdresser's perspective. In a typical barbershop you have an average of X people working at any given time, you know that an average haircut takes X minutes etc. Factoring in things like peak working hours, hours worked in a day, days worked in a week... will give you a surprisingly accurate estimate for the number of haircuts sold by a barbershop in a week. This can then be expanded for the whole of the USA by using step 3...
3. Use Proxies: As you mentioned, proxies are very useful to have banked in your head. Ultimately the interviewer wants to see a logical breakdown of your estimation and so personal experience is an excellent tool to have at your disposal. I like to have memorised the populations of major countries/capital cities along with an example town/city that I am very familiar with. Using the same example I would know that in an area I live with 20k people, there are X barbershops. I could then extrapolate this for the whole population of the US.
4. Replacement Concept: Often for estimating the number of products sold in a market annually you can use the life expectancy of that product. E.g. If there is 7.4B people on the planet and average life expectancy = 70 years - we know that (with 0% growth) in 70 years there will be another 7.4B people. i.e. 105M born a year. We can also then factor in growth rates for growing markets.
5. Spacial Awareness: Some of the most abstract (and rare) problems I have faced involved testing a candidate's spacial awareness. E.g. "How many tennis balls can I fit inside a Boeing 747?". I think the best techniques here are to use the pen and paper you are given and take each dimension one at a time. You may find it easier to estimate the distance between rows of seats and, drawing on experience, estimate the number of rows in a plane rather than guessing the length of the plane.

You can see how some of these techniques overlap and draw on each other. Luckily these types of problems can be easily practiced in your own time and remember that a logical thought process is much more important than an exact answer.

Hope this helped!