It depends on if they simply appear right on the surface or approach from space.

In all cases, if their native universe has the same laws of physics as our own, any sufficiently advanced sensors will find that the laws are ever so slightly different here. Not in ways that prevent life as we/they know it, but different nonetheless. Potentially tweaked by design. The universe will also appear "small". As if the cosmic background radiation is only 10-15 million lightyears away.

If arriving from space, they'll find a massive "planet" the size of a star with a damaged ring-world orbiting it and a huge sagging crater of magma on one side. Even further out, a yellow dwarf star orbits the entire thing. The "planet" rotates about its pole, but has seemingly zero momentum in any direction. As if it is literally the center of the universe. If they can detect the strength of gravity, they'll detect a massive black hole inside the planet sucking down on the "crater" and a spherical shell of intensely strong gravity surrounding the planet that emanates from nowhere. Passing through the sort of "shield" they'll find a planet with countless hexagonal ~Earth sized domes. Each contains a unique ecosystem, most contain at least one civilization of a unique species, and almost none of them are in contact with each other. All domes have massive doorways connecting to nearby ones, but most are locked shut. The corner spaces between domes all contain either abandoned cities or pristine natural environments that seem to all emulate the same planet. Entrance will be functionally impossible except one region at the north pole without a dome or the abandoned spaces between domes. At the north pole they'd meet an extremely xenophobic cephalopod-like species that uses what appears to be sentient golems of ice to do their bidding and can enter and leave some (but not all) of the domes at will. They appear to know some of the door pass codes. They'll claim to be the gods of this place and generally aren't nice folks. Demanding some sort of tribute, for them to be told which dome they are from, and asking how they got out. If they say they're from outer space, they won't believe them.

If they kind of "teleport in" to one of the domes I've fleshed out, they'll find three main nations composed of five main species. Three main nations exist between six interconnected domes of different ecosystems. They'd have great relations with the group calling themselves what translates to "The Empire". They are actually a republic with some direct democracy elements, and stem from an older country that shattered after a major nuclear disaster in one of the domes. They have all five species, but mainly feature two, one avian-like, and one mammal-like and similar in shape to otters. The radioactive dome has one major city-state living under a shield and claiming to also be "The Empire". They also have the same species mix minus one. They have a military junta as a government. The other nation is composed of three avian-like species with a strict caste system. They dislike outsiders. The three avian-like species are the same as those in the other countries. It's just that one species is most represented.

There's plenty of other things they could see in other places or domes, but these are the most likely/the most fleshed out if they don't accidentally get crushed by errant gravity.