Unity is the game engine I chose for the game. The most feasible way for me to enable mod support, was to make tools for Unity itself. They are not standalone, you have to download and install Unity in order to make mods for Orchid Rain. I'll explain here the best way to do that.
There are two ways to download Unity. The first one is the simplest. Just go to this link which will download the Unity 2018.4.12 installer directly from Unity's servers. If you're not so sure about direct download links, you can go to the Unity downloads archive and download 2018.4.12 by yourself; just be sure to choose the 64 bits version to avoid potential compatibility problems.
Look for the dropdown next to the green download button in the Unity 2018.4.12 row. Expand it and click on "Unity Editor 64-bit".
I'm installing a different version of Unity here, since I already have 2018.4.12. Also, you will have to create a Unity Account at some point.
The second method is to use the Unity's official download and installation manager, called Unity Hub. This approach requires the most steps, but it's the way you're meant to download and install Unity. It's good for managing multiple versions of Unity, as new versions don't replace your current installation, but are installed separately, and with many versions, it could be a bit messy. Nevertheless, you should stick with 2018.4.12 to be extra safe and minimize the possibility of your mods not working properly, as that's the version of Unity I'm using to make the game.
To download Unity Hub, go to the link provided previously and click on "Download Unity Hub". After that, install it like any other program. You might have to create a Unity Account.
Since 2018.4.12 is not the latest 2018.4 build, the Unity Hub doesn't show it by default. You will have to go to the download archive and download it with the "Unity Hub" button. To find the right version, click on the Unity 2018.x tab, and look for Unity 2018.4.12.
I recommend that you don't check any of the boxes, as you are not going to need any module for modding. Just click on "INSTALL".
Unity will start downloading, and it will install automatically.
I've prepared a Unity project with everything you'll need to start modding, that way you don't need to create a project and set it up yourself. To download the project, head to its GitHub page, hit the "Clone or download" green button, and then click on "Download ZIP".
Download and extract the zip contents on a directory of your choosing.
If you used the first method to install Unity, you will see this window when launching the executable. To open the project, click on Open.
If you installed Unity using the second method, open up the Unity Hub and press the "ADD" button.
Unity projects don't have the usual file you double click to open. Instead, you have to look for the project folder, and "open" the project from it. To be precise, go to where you extracted the zip file, go inside the "OrchidRain_ModdingTemplate" folder, and you'll see 5 folders. Among them, one is called "Assets". That tells you you're in the right folder. Don't select any of those 5 folders, and then press the "Select Folder" button, on the bottom-right.
When you do that, you'll see the name of the project (OrchidRain_ModdingTemplate) appearing on the list. Click on it and the Unity Editor should load.