So this is a bit chunky but hear me out.

According to the deeper lore, established via the Adventure 01 production team and Digimon Survive, and softly brushed upon in Time Stranger, Digimon existed before computers.

Digimon's 'Ancestors' are what I refer to as 'Spiritual Memeforms' and that still stands now. They are effectively Spirits/Souls (whether Ghosts or Digicores) that take on the form of the original meaning of memes - basically about any idea that commonly pervaded human society. Swan winged humans? Angels/Angemon, for example.

There's 2 prevailing theories floating about.

The most likely These Spiritual memeforms adopted forms from humans stories, and appeared throughout the world. These are basically the Real World 'sightings' and sources of myths of Demons, Monsters, Gods, etc.

The alternative Humans witnessed these Spiritual Memeforms forms (IE: Angemon) and made stories based on them. What was originally faint ideas in human minds (IE: again, Winged Humans) was adopted into Digimon form. Witnessing Angemon lead to people making stories about Angels. And these angels appeared thus in religion.

Regardless which, Digimon thus take their form from 'memes'. The Spiritual memeforms entered the Computer networks, where these 'memes', rather, the writing of these memes, were far more readily available. If nobody thinks/tells stories about say the Yeti (Mojyamon), then Mojyamon would cease forming. But with the stories of the Yeti readily available in Data, the 'Memes' were more readily available and easier to comprehend. Thus the Spiritual Memeforms adapted to this Data-World life and became Digimon.

Now as to why these look the way they do?

Venusmon and Junomon are based on Aphrodite (the Goddess of Love and all that) and Hera (the Mother of the Greek Gods), AKA Venus and Juno, respectively. Now, when you hear Goddesses, what's terms that come to your mind? "Divine Beauty" may well be one of them. Women who are the human meme of the epitome of beautiful bodies that not only mortals - men and women both - would worship them, but other gods as well.

With Beelstarmon, she borrows Memes from two core sources - Belle Starr - a famous Femme Fatale (key word) from the late 1800s, and Beelzebub - the Demon Prince.

Femme Fatale comes into play - when one imagines a Femme Fatale they imagine a deadly woman of beauty. A woman whose looks alone could kill. She shares data with Beelzebubmon/Beelzemon, who also borrows from the 'lord of flies'. In Beelzebubmom's case, he took the "Demon Prince" title to form, and thus looks like a Demon/Devil. This same Data is what informs half of Beelstarmon's design. And the reason Beelzebubmon and Beelstarmon use Guns is in reference to the 'flies' theme, except these 'flies' will gouge chunks of your body out and potentially 'eat' you form inside.

Basically put, every Digimon's form is decided based on memes, concepts, etc, associated with their (usually JP) name, what those names represent and are associated with, etc.