I don't know. Black Holes are infinitely confusing and there exist several solutions to what they might be like inside that event horizon, depending on how you define your 'coordinate system', and, what type of Black Hole you're describing. The truth is that we have never even seen a event horizon, although there are some experiments with light that is expected to be close analogues, maybe?

The simplest solution is the one for a non rotating black hole.

"It is spherically symmetric - a so-called Schwarzschild black hole, after Karl Schwarzschild who, in 1915, was the first to write down the equations defining and describing such a black hole; a special solution of Einstein's equations of general relativity. (However, it took physicists more than fourty years to come to understand the weird spacetime geometry that Schwarzschild's equations imply!) As has already been mentioned, this type of black hole is eternal - it has always been there, and will always be there. More realistic black holes with a definite beginning (for example those produced by the collapse of a massive star) or eternal black holes which rotate all have a somewhat more complex inside structure."

As for if we know what they have inside a event horizon?

In my book 'times arrow' is of one direction. If I assume that a black Hole can accrete mass by 'gravity' and that the event horizon can grow by that, then I don't see how times arrow can behave differently inside.

If I assume that 'gravity' exist both inside as well as outside the event Horizon, and attract, then they are not 'outside' this universe in any way.

If I assume that a free falling observer can pass the Event Horizon, in a for him measurable time, then the 'apparent Event Horizon' doesn't define something 'closed', instead it seems more of a 'one way street', that we all can take. The thing we can't do though, is to get our observations back, to outside that Event Horizon.

That said, here's a 'easy' description/explanation for a Schwarzschild black hole. The problem is the way mathematics freely throw around descriptions like 'negative time', and in this case, time and space changing 'axis' without explaining how they get to those ideas. It gives people a lot of strange ideas, not validated anywhere, except in the mathematical definitions and theory's describing them.

Changing Places.