As @Dood points out, this is unfortunately not the case with many post 2012 Apple laptops (and many models of Mac Mini post 2013 and iMac post 2013 too - some 27" iMacs of the same date CAN be upgraded).
I intentionally went for an older MB Pro so I could make further changes down the line. That meant I had to go for a 2012, non-retina machine. i7, 16Gb RAM, 256Gb SSD and 1Tb drive replacing the DVD drive. It's heavier than I'd like, can't support Mojave (even though Apple say it should), is about as quick as it can possibly get already and it's certainly not as sleek as newer offerings, but it's blisteringly quick for a machine of its age, much quicker than my relatively new i5 windows laptop at similar tasks (i5, 8Gb, 256Gb SSD). That's an apples and oranges comparison, but there's 6 years between them!
I'd echo others in recommending you buy the absolute best spec you can afford.