Samaritans are not Jews.
R Yosef Eisen at chabad.org explains why
When the Assyrians exiled the Ten Tribes, the conquerors brought in a
foreign people called Cutheans to populate the vacated territory.
These people were idol-worshipers, and G‑d sent lions to decimate
them. Out of fear of the lions, the Cutheans converted to Judaism, but
the rabbis of the Talmud debated whether their conversion was valid or
not. The Cutheans’ Torah observance was spotty — extremely strong in
some areas, but very weak or nonexistent in others.
Settling in the Samaria region of Eretz Israel, over time the Cutheans
became known as Samaritans. Fearing that the Jews returning from
Babylonian exile would reclaim their ancestral lands, the Cutheans
became bitter enemies of the Jewish people, even going so far as to
attempting to sabotage the construction of the Second Temple.
Disguising their evil intentions, the Cutheans offered to help build
the Temple. Realizing the Cutheans’ real goal, the Jews rebuffed their
proposed aid. Stung by this rejection, the Cutheans convinced
Ahasuerus that the Jews wished to foment rebellion against Persian
rule, so he suspended construction.
On a number of occasions during the Second Temple Era, the Cutheans
were an anathema to the Jewish people. Finally, when the rabbis of the
Talmud discovered that the Cutheans were worshiping idols and not
keeping the commandments, the sages expelled the Cutheans from the
Jewish fold and declared them to be gentiles. Although most of the
Cutheans eventually died out, a small group exists today, living
around Mount Gerizim in Israel. While they have ancient scrolls that
bear some resemblance to the Five Books of Moses, these descendants of
the ancient Cutheans have no connection to the Jewish people.
Ohr Somayach explains further
The Samaritans were non-Jews brought to Israel by the Assyrians to
populate the North after the exile of the Ten Tribes. They ostensibly
converted to Judaism, but in reality they continued worshipping idols,
save for a period when they were mistakenly considered genuine
converts; hence the Samaritans were not considered Jews, neither by
Jewish law nor by the Jewish people.
They did not accept the Oral Tradition, which forms the overwhelming
bulk of Jewish law. They also did not accept any books of the Bible
except for the Pentateuch and the book of Joshua. Today, the Samaritan
version of the Torah manuscript differs from ours by about 800
The Samaritans often acted as enemies of the Jewish people. They tried
to destroy the Temple and to inform against the Jews to Roman
authorities. The parable of the "Good Samaritan" was actually an
anti-Semitic story intended to discredit the Jews.
For references in the gemara see e.g., here and there.
As to whether you can pray in their house of worship, you do not say if you mean it as part of an organized prayer service (very likely forbidden by all) or alone. Circumstances are important as one issue at play is marit ayin (giving the impression their religion is authentic Judaism). Since their religion is not avoda zara1 it could possibly be permitted to pray there alone (see bottom of here and bottom of here) but it is a halachic dispute and you need to CYLOR.
1 To be sure, I asked Aaron Shaffier (also here on MY), a religious tour guide who has been many times to Har Gerizim. He wrote that "As far as anyone can tell they are complete monotheists".