It isn't. Well, not in the easy, user-friendly little hatch way like the 27" non-Pro iMac has, and that's weird/unfortunate. The 21.5" and the 27" Pro are sealed shut to regular mortals, but the regular 27" model retains the little access door. There's no consistency/rhyme or reason on this so you can't chalk it up to a "pro vs. not" issue
or a size (21.5" vs. 27") one. You'd think a 27" iMac with "Pro" in the name would at least be as RAM accessible as its cheaper, non-Pro cousin.
But Apple still sells notebooks with "Pro" in the name that come stock with 8GB RAM (even when the larger one comes with 16GB...what's that saying? That the 16" is
more Pro-er/deserving than the 13"?), so, again...just weird, arbitrary specs across models of the same family, etc.
This is the kind of stuff I wish Apple would address in 2020. There should be 16GB stock in all MacBook
PRO models, regardless of size, and either make the entire iMac lineup's RAM user accessible again (including the 21.5"). Or, at the very least, make the iMac Pro easily upgradable. At least at that point they can say "well, we have more wiggle room in the 27" models to make that happen; and that's a nice little perk you'll enjoy by stepping up to the larger models"** or whatever. They've allowed it in a much-cheaper 27" model for years, even after the Faux Thin from a Certain Angle™ redesign of late 2012 (seven-plus years ago!
) where user access to the 21.5" was done away with.