>How do fishes get into isolated inland lakes in the first place?

For random small ponds and lakes, there are two main ways that fish get into seemingly isolated bodies of water: people, and flood events.

I cannot emphasize this enough, people are absolutely obsessed with putting fish in all sorts of bodies of water. Fishermen want fish to fish for, and nobody wants mosquitoes. People go to extreme lengths to get fish into everything from their farm ponds to remote lakes in the middle of nowhere in the mountains. In the old days, this meant packing milk cans full of trout fry out on mules, nowdays it means airdropping them.

If the fish you see is any sort of trout, bluegill or other freshwater sunfish, a bass, common carp or goldfish, golden shiners, fathead minnows, mosquitofish, or tilapia, there's a very good chance it was stocked in the body of water by a person. These fish have all been spread all over the place outside of their native range by people. (this is a North America-centric list, other places will have their own commonly stocked fish)

The second method of fish dispersal is flood events. When there's a big flood, all the water flowing into an "isolated" pond has to flow out. The exit may not be obvious most of the time, and it may not have water in it most of the time, but during floods there's a lot of water moving on the land and that lets fish move around to places you wouldn't expect. There are relatively few isolated bodies of water that are truly isolated. This is even more true if you take the long view...there were enormous floods at various points toward the end of the last ice age, and as recently as the 1800's we've had megafloods in the central valley of California, for instance.

Now, I know people always bring up birds, but I'm not at all convinced that is a major method of fish dispersal. The first option is a bird carrying an adult fish and dropping it in a new lake. I find this an extremely unlikely method of fish dispersal, because nearly all fish are external fertilizers. Which means you need a male and a female to be present, you can't just have a pregnant female carrying fertile eggs. The odds that birds would happen to drop two separate fish in the same pond, after carrying them (and you'd have to carry them alive a long way) are just too small for it to happen regularly.

There's a better case for birds consuming fish eggs, but it's still only proven for a few fish species that the eggs can survive passage through a duck digestive tract, and a great many fish don't lay eggs in places where birds might eat them anyway. Probably happens sometimes.

But it doesn't happen very often, and we know that because historically there have been a lot of lakes without fish in them. Especially in high mountains or areas in the north, lots of lakes just...didn't have fish. These often housed unusual or unique ecosystems with amphibians and insects found in few other places. And also it's very common for fish to be found in one watershed and not neighboring watersheds, even if other fish lived in those other watersheds. So it's clear that most fish aren't getting airlifted to nearby streams or lakes, because we know they just weren't in those streams or lakes.

Of course, as I mentioned before, people are absolutely mad for moving around fish so nowadays nearly all of those previously fishless lakes have fish in them thanks to historic stocking efforts.

>and why don't we see more divergent evolution / speciation given the separation of each group of fishes from each other?

Ponds and lakes are usually very short lived, on a geological or evolutionary time scale. Ponds usually come and go over the course of a few hundred or few thousand years, they just don't stick around long enough for speciation to happen. Lakes come and go too, the Great Lakes only appeared at the end of the last ice age, for example. And of course many of the "lakes" people know about are actually reservoirs built by humans in the past hundred years or so and stocked with a mix of human chosen fish.

Old lakes do have a bunch of unique species: the rift lakes in Africa and Lake Baikal are two excellent examples of this phenomenon. And in general, freshwater does cause a bunch of speciation...there are almost as many freshwater fish species as marine species, despite the total volume of freshwater habitat being enormously smaller. This is because isolation in different watersheds causes speciation. But you won't necessarily see that diversity because if you are fishing, you are probably catching and seeing the very handful of fish species that people have stocked in ponds and lakes all over the place.