Unity can be obtained at
The Personal version is perfectly capable and is free to use for non-commercial or people learning, or anything under a certain amount of income. It's quite a benevolent giveaway.
There are actual commercial versions, which let you do more things, give access to cloud building and let you remove the start up logo. But for our purposes they really aren't needed.
Also, by default, Unity installs "MonoDevelop" as the editor for C#. You don't need to use this, I use Rider from JetBrains, as it is more akin to IntelliJ.
It in itself has its own licensing costs though, and only has a limited 30-day trial, unless you have the all products pack and have access to it from that?
Or, you can use Visual Studio, from Microsoft. Which is free.