Googling issues like this tends to lead to reading lots of pop culture myths about them that are easy to find on mass market sites.
Move past myths about druids and Irish festivals of the dead and the actual history is somewhat different.
Samhain wasn't a festival of the dead, although end of (agricultural) year/pre-winter is a fairly natural time to think about spirits or death/mortality due to winter being a deadly season in pre-modern times.
Christians have pretty much always celebrated saints and martyrs, and days for all saints moved around the calendar. Samhain is unlikely to be the origin of the current All Saints Day though as the tradition for that date started in the Germanic world, not the Celtic.
Ronald Hutton:
Charlemagne’s favourite churchman Alcuin was keeping [All Saint Day on 1 November by 800 AD], as were also his friend Arno, bishop of Salzburg, and a church in Bavaria. Pope Gregory [IV], therefore, was endorsing and adopting a practice which had begun in northern Europe. It had not, however, started in Ireland, where the Felire of Oengus and the Martyrology of Tallaght prove that the early medieval churches celebrated the feast of All Saints upon 20 April. This makes nonsense of [the] notion that the November date was chosen because of ‘Celtic’ influence
So unless the German Christians were ripping off an Irish festival as a marketing ploy while Irish Christians were completely oblivious as to the plan, Samhain is not the origin.
The answer is the same for all of these “X is pagan” myths, there may be some cultural commonalities as is to be expected, but these are largely overstated and do not equate to pagan origins.
The idea that unless something appeared entirely out of a vacuum then it must be “pagan” is inane.