As far as I'm aware, it connects with WS3D and RtCW through B.J (not the MachineGame reboots, though).

It's a prequel to Wolfenstein 3D

https://wolfenstein.fandom.com/wiki/Wolfenstein_3D:_Spear_of_Destiny

Spear of Destiny and its expansions are a prequel to Wolfenstein 3D and it's expansion (strictly speaking SD is not much more than an expansion of W3D itself) neither game has any real connections to the two previous games in the series Castle Wolfenstein and Beyond Castle Wolfenstein.

They both only have vague connections to any of the future games, for example Wolfenstein The Old Blood's first half is essentially the same plot as the first episode of W3D so you could say that it's a reimagining, or that they both happened, or whatever else you want to say since it's never clearly stated only lose allusions are ever made, just enjoy the games and the lore will build up as you play.

The lore is you gotta kill some nazis.

man i still haven't played this.

In a nut-shell,
the nazis seized many priceless artifacts, including religious artifacts.

The "spear of destiny" was one such sought after treasure. Basically, it was "the spear that pierced the side of Christ."
It's a lot like the "holy grail" as well, which was supposedly held out to catch the blood of Christ.

So in this sequel, along with just slaying nazis, you are also out to recover this sacred relic from the evil empire that possesses it.