Where is [Spoiler]?

[ObsidianFire]

The 13th Shard is broken on a fundamental level. So how the 13 Shard works probably isn't how the rest of the Shards work. However, even Voidsent follow a very similar process to the Ascians when they come to the Source.

Voidsent can't make a portal to the Source without someone on the Source summoning them. They don't just travel to the Source randomly. Once they are on the Source, all Voidsent have to posses some kind of physical form from the Source so they can interact with it. Different kinds of Voidsent like different kinds of bodies. So you'll get everything from Voidsent possessing eyeballs to Voidsent possessing sculptures to Voidsent possessing humans. "Killing Voidsent" isn't actually killing Voidsent; instead, we kill the physical form they are possessing which sends the voidsent back to the Void.

The "portal" we used to get into the 13th Shard is a result of Xande making a pact and trying to summon the Cloud of Darkness. Only the portal for a Voidsent that powerful required so much energy from Dalamud that the Crystal Tower couldn't contain it and caused the 4th Umbral Calamity in the process. And the goal of that was for the Cloud of Darkness to absorb all the aether from the Source to destroy it. At the end of the Crystal Tower raid, Xande's pact with the Cloud of Darkness is undone and the portal is finally shut.

In order to get to the Void, we'd probably have to re-summon a Voidsent on par with the Cloud of Darkness again and then somehow find enough energy to open a portal to the 13 Shard. Only... the last time that was done... we got an Umbral Calamity in the process...

[Kalise]

Voidsent can't make a portal to the Source without someone on the Source summoning them. They don't just travel to the Source randomly.

While many Voidsent are summoned to the Source, not all of them are.

There's things such as during the SB WHM quests where Voidsent portal themselves in to try and feed of the Padjal's uncontrolled Aether. There's places like Haukke Manor which become infested with Voidsent without anyone actively summoning them (Unlike, say, Tam-Tara hard), you have open world areas such as Northern Thanalan where Voidsent appear and gather seemingly at random.

This suggests that Voidsent CAN come to the Source of their own volition, though, I suspect there's some limitations to it, such as them being unable to CHOOSE to come to specifically the Source and instead just sense high levels of Aether and go to it completely oblivious to what shard it is on. Meanwhile summoning would be more direct travel, allowing specific entities to be brought to specific shards (As seen by the many times people summon big bad demons to fight against us)

As far as the "needing to possess a body" thing goes, I'm a little confused on that. Since, for example, in BLM and RDM quests, there are times when enemies open up portals and giant Chimera's come through. With no evidence of having any bodies around (Certainly not giant Chimera ones). I mean, these instances of summoning suggest that these Voidsent are coming through the portals body and all (Though, I suppose it could be a case of artistic licence where devs didn't feel the need to keep putting various bodies around the place for Voidsent to inhabit)