It's common for shonen anime series to gloss over minor details that are unimportant to the plot. This helps a story maintain its pacing and avoids distracting viewers from tedious world-building details that aren't really necessary, such as highly personal issues like reproduction. Most anime characters are human, and it's obvious how they were born, but Bleach has both flesh-and-blood humans and supernatural characters.
Despite the fact that Ichigo's father, Isshin, was once a Soul Reaper, he acquired a physical body, a gigai, and had three children with his human Quincy wife, Masaki. Meanwhile, given their distinct male and female physical forms, some Bleach fans have wondered if Hollows and Arrancars can reproduce as well. Although the "Hueco Mundo" story arc glossed over this, fans can't help but wonder if Hueco Mundo has witnessed the birth of Hollow and Arrancar children.
Arrancars' Implied Reproductive Capabilities
Hollows are created when a Plus is bound to the mortal plane for personal reasons, such as a lingering grudge or unfinished personal business. Once its Chain of Fate is gone, a hole appears and the Plus mutates into a Hollow, complete with an animalistic form and a bony white mask. However, Hollows can remove their masks and assume a more human form as Arrancars, with the most humanoid and stable Arrancars forming when Sosuke Aizen used the Hogyoku on them. These Arrancars regained their original human minds and human anatomy. Hollows and Arrancars are only known to be made, not born, but Bleach lore still vaguely suggests that Hollows can sexually reproduce.
All known Arrancars have a distinctly male or female form, and with personalities and personal pronouns to match, meaning they are not just things; they are people, even though they are not Pluses or humans. Arrancars are evidently human in form simply because they were once human, and that includes reproductive anatomy. The main example is the 8th Espada, Szayelaporro Grantz, who is confirmed to have his Hollow hole not on his chest but on his genitalia -- though of course, the Bleach anime doesn't directly show that. While this was for humor's sake, it's still telling that at one point, Szayelaporro honestly thought that Pesche, another Arrancar, was holding his own male anatomy during a battle, only for that phallic item to be the handle of Pesche's sword, Ultima. There must be a good reason why Szayelaporro believed Pesche was doing something inappropriate.
A more subtle example is the fact that female Arrancars, such as Nelliel and Harribel, have secondary sexual characteristics -- in other words: they have breasts. On a meta level, those female Arrancars may look that way just to visually appeal to anime fans, but in Bleach lore, it may be because Arrancars really can reproduce, including nursing young. However, this is unconfirmed, and it's equally likely that Arrancars have male and female reproductive anatomy simply out of habit. They were all once human beings who of course were biological beings capable of reproduction, and their Arrancar forms reflect that.
Perhaps when they were exposed to the Hogyoku and lost their Hollow masks and forms, the Hogyoku didn't know what else to make these Arrancars look like, so they defaulted to what they looked like as living humans, including reproductive anatomy. If that's the case, either they can reproduce and Bleach just doesn't show it, or they can't and their sexual organs are just a relic. It may be yet another mockery of Arrancars, with these twisted spiritual beings attempting to find meaning and happiness in life and emulating humanity in many ways, despite the fact that they are fundamentally different and cursed. They are simply lost souls, no matter how human they appear.
Soul Society Reproduction and Family Units
Members of the Soul Society, on the other hand, more closely resemble human society and family units. The Soul Society lives in the ideal spiritual afterlife, where no one is a cursed mockery of what humanity is supposed to be, distinguishing them from Hollows and Arrancars. This includes family and reproduction, where things are clearer and more familiar.
Families can be formed in two different ways in the Soul Society. One way is for dead souls to arrive there and form tightly-knit found families, where adoption is common. Rarely do complete human families arrive in the Soul Society together, so the scattered members of a family may arrive at different times and places and join various found families. Marriage is also an option, with the impoverished Hisana somehow convincing Byakuya Kuchiki to marry her. Fan theories suggest that Hisana and Rukia died at the same time on Earth and visited the Soul Society together, meaning that even newcomers to the Soul Society can do more than create found families -- they can marry someone and likely have children with them.
Most of all, native souls in the Soul Society are definitely capable of sexual reproduction, while it's far more ambiguous for Arrancars. The four noble houses rely on that, with people like Yoruichi Shihoin and Captain Byakuya Kuchiki being born into those families as official heirs. Bonus Bleach materials make this even clearer, with Byakuya's late father Sojun bearing a strong resemblance to him. The members of Marechiyo Omaeda's family certainly resemble him as well, practically confirming that they are "biologically" related and not only a found family. It makes sense that in the inspiring, relatively relatable Soul Society, families are important and clearly defined, including reproduction, while it's ambiguous and strange in Hueco Mundo, the land of the restless dead.
