The term Jew as we now receive it in either English or Greek forms was capable of being applied in several ways, including (roughly) meanings corresponding more to tribe, to all the 12 tribes, to the religion, to the wider culture inclusive of its religious elements, to a geographical origin, etc.
It seems quite clear in Paul that he is speaking of the Jewish people as a whole (from all tribes and those who joined them)...
... because Paul emphasizes God’s covenant (as continuing, not having been terminated or abandoned or superseded, etc....that being his key point) ... and that covenant exists (and existed then) with all the people we would call “Jews”..... not just the Jews from Judea and not just the Jews with tribal identification as Judeans, but including the Jews from the north such as Nazareth (Jesus’ community, for instance) and the Jews from the disapora of that age, such as Paul himself and such as all the Jews in the several synagogues where Paul went or visited or preached around the eastern Mediterranean... indeed his epistle to the Romans was in response to a situation he observed in those diaspora synagogues .. so Paul was certainly not limiting his teaching to just the Jews who were of the tribe of Judea, or who happened to live in the little territory around Jerusalem.... Paul was plainly speaking of the entire Jewish (as we now call it) faith tradition/belief and people were thus part of the Jewish (as we use the term) covenant.
(Jesus understood the same, as when he rejected the pagan woman’s pleas for intercession.)
Christians are “grafted onto” the Jewish people by virtue of faith in the “Jewish God” as revealed by Jesus Christ (Himself of course a practicing Jew in this world).
Christians are not merely grafted onto one tribe of the 12, but the entire People Israel and their collective understanding(s) of covenant, with its various permutations perhaps, but the entirety of the Jewish People’s ongoing, continuing covenant with the Divine.
or something like that. That’s pretty close for sure...
PS: St Paul was not only a Jew but he was very proud to be one
.. it meant a great deal to him...