The only reason people compare them is that they both incentivize "throws." But wrestlers and judoka have very different definitions of that word. For wrestlers, a throw is a takedown where you lift the opponent off the ground with your upper body. For a judoka, it's much more broad; every takedown in the canon of judo is a "throw," including trips and even leg grabs.
This distinction might seem academic but it matters pretty much immediately when we start comparing judo and Greco. Trips and sweeps are huge in judo - they're the primary way we threaten the opponent to influence their movement and stance. They're not allowed at all in Greco. Two of the most popular and high-percentage "finishing" moves in judo (uchimata and osotogari) are leg throws. Most other popular moves that aren't classified as leg throws still really on a leg assist (haraigoshi, taiotoshi). The most popular arm throw (morote seoinage) doesn't have any leg contact, but it requires a gi. Out of the top 15 standing moves in judo, I can only think of three that would be legal in Greco: ogoshi, ippon seoinage, and ura nage. Furthermore, a judoka's throw is only scored if the opponent lands on their back or side, whereas in Greco, you can qualify for a score even if the opponent lands on their stomach
And that's before we get into newaza, where the differences are similarly extreme. The obvious one is that judo allows subs while Greco doesn't, which is a pretty enormous difference. Almost as importantly, you can score a throw on a grounded opponent in Greco, whereas in judo, a grounded opponent cannot be re-thrown for further points. Judo also has a different definition of a "pin" than Greco does
Ppl commenting that newaza are just ground techniques is correct.
However, what op is asking is what is the difference in bjj newaza and judo newaza.
The answer is the ruleset.
In judo you can win by pin or submission on the ground and the refs will be very quick to stand you back up. So the ground game that comes from those rules is extremely fast, scrambly, and heavy heavy heavy pinning, much less technical details, and less submissions. There is no focus on guards. When a judoka gets into a bad situation on the ground they will most likely belly down or turtle and wait for ref to stand them back up. So the ground game in judo is very focused on turning over ur opponent from belly down or turtle.
In bjj, the time in the ground is unlimited and you cannot win by pin. The goal is submissions and to be able to submit your opponent at the higher levels the small minute details become extremely important. The pace (after passing guard) is also slower because the ref will not stand you back up. There is also a huge focus on guards and passing guards. Belly down is non existent in bjj because unlike judo when you belly down in bjj, ur opponent is allowed to sit on top of you and crush you for the entire match. They're also allowed to pull your head up by ur face (illegal in judo) and choke you the fuck out.
In short, bjj has much more technical, intricate and advanced newaza. Judo has a much more aggressive and powerful form of newaza.