This is because until the arrival of the technology to freeze meat, and neatly-wrapped supermarket produce, the animals were kept alive until just before they were sold.
So many butchers would have a slaughter house close to their butcher's shop. A larger butcher's enterprise could have had a farm where cattle were grazed and fattened for market, slaughtermen working in the slaughterhouse in one of the farm's outbuildings, and what we think of as butchers working in the shop in a market in town. The term butcher can refer to either jobs in the 19th century.
The ability to freeze meat and a growing understanding of the way that diseases can be passed - led to a separation of the slaughterhouse from the butcher's shop - and the distinction between wholesale and retail meat trades. This happened gradually during the 19th century, with the first attempts at shipping refridgerated meats (rather than live animals) from Australia and Argentina in the 1870s. Meat inspectors were introduced in the early 1900s, but well before then there was a lot of regulation around slaughterhouses, not so much because of the health hazard (which was not well understood), but because of the unsavoury sights and smells !!
I'm not sure exactly what training a butcher's apprentice might undergo, but I think there must have been a progression from slaughterman to butcher, as I have come across a number of examples of young men starting as slaughtermen working for a master butcher, then being "promoted" to a butcher with their own market stall. Certainly it would require a good understanding of the anatomy of the animals, which would be good grounding.
Hope this helps
All the best
Stephen Foote
