CircuitBlocks runs in your browser at code.circuitmess.com, so there is nothing to install. What matters is not which operating system you use but which browser.
You need a desktop browser that supports Web Serial: a Chromium browser (Google Chrome, Microsoft Edge or Opera) or an up-to-date version of Firefox, on Windows, macOS, Linux or ChromeOS. Web Serial is how the page talks to your device.
Safari will not work. It does not support Web Serial, so it cannot connect to your hardware at all. Older versions of Firefox will not work either, and CircuitBlocks will tell you if yours needs updating.
Phones and tablets are not supported. CircuitBlocks needs a desktop or laptop.
What you write in it: you can drag blocks together, or switch to Code mode and edit the same program as MicroPython source. CircuitBlocks writes MicroPython for every device it supports. If you want C++ instead, that is the Arduino IDE or ESP-IDF route — see What programming languages can I use with CircuitMess kits?
If your device will not connect, check that your browser is up to date first, then message us here, or email [email protected], with your device, browser and operating system, and we will work through it with you.