Welcome!
Lemmy will be different from Reddit, sometimes in good ways and sometimes in annoying ways (mainly because it's still being built, there are a lot of people working on the code and planning behind the scenes). Feel free to make more posts like this as you think of questions or have issues.
For the communities themselves, try searching here: https://lemmyverse.net/communities
I didn't see anything for Koalas, but you could try things like "nature", "animals", etc.
Which brings up the other point, make the community! Lemmy being new is both good and bad. There is not that much content to view, but any content that is created will be seen by a lot of people. Make a community and teach people about them, get people posting.
Here are a few others you might enjoy:
- !environment@beehaw.org
- !wildlifephotography@lemmy.world
- !biology@mander.xyz
- !australia@aussie.zone
- !environment@aussie.zone
- !climate@slrpnk.net
Also you can totally make that image (or another image) your profile photo here. Click your name in the top right to edit that stuff. I myself like sea otters :)
If you want some browser extensions / apps, here are lists: