My previous job wasn't really setup to do remote work but we had a second team working almost 12 hours opposite (Romania vs Kansas). The "offshore" team was paying a heavy tax because everyone was working at the office, so we had a ton of hallway conversations that never got documented anywhere they could access it. Even though everyone knew this, because we were co-located it was extremely difficult to overcome because it's just so easy to not document things. When your entire team is 100% remote, you are forced to document things a lot more since there aren't hallway conversations unless you specifically video chat with a single individual.

How do you handle discussing complex code, data structures and such concepts over online? In such cases, I find that being in the same room and using a white board makes it easy to communicate across. Maybe some programming domains that type of issue if less common?

Sometimes (not too often) I do such things remotely.

In my case, voice call + screen sharing works OK. On the screen sharing I can use whatever software I find adequate, be it Word, Visio, Paint.net, balsamic mockups, etc.

As a side effect, we can continue working on those documents/diagrams/whatever after the discussion, e.g. include it in the documentation. Much harder to achieve with a white board as someone needs to digitize those drawing. In my experience working offline with whiteboards, no one actually doing that. In the best case, a digital photo is kept somewhere no one will be looking, in the worst case the board is just wiped clean.

Some kind of screen sharing app with Krita = virtual whiteboard. You have a lot more room than a real whiteboard offers, since Krita has a "pseudo-infinite" canvas, so you don't need to erase things. And you can save the file for reference, or to send it other people.

In some cases people literally point their camera at a whiteboard!

You laugh, but it works really well to do this so long as you remember to stand out of the way of the camera.