Plus it comes with it own bloatware and crap bundled + several apps on playstore have specific warnings for samsung users that such apps have be configureddifferently for the device unlike other brands where its simply install and forget.Both realme and Samsung are at their own circumstances, you should go for Samsung, its features are best and it have a large number of various models which are in the budget of anybody.
Fingerprint sensor and cameras are garbage on budget Samsung phones but I'd still prefer them over BBK and Xiaomi trash anyday.
How's the network reception & call clarity of M21 ? I have read in many reviews about network issues in Samsung M series phones.Fingerprint sensor on the M21 is actually very good. I needed to (at first) press firmly but now after a couple of hours it seems to unlock effortlessly and quickly. Maybe my fingers were damp at the time or maybe it's because I put on a back cover now and the angle of my fingers is working better now.
Camera I haven't used yet and don't really care but the youtube video reviews show pretty good pics even in dim lighting.