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 Chromium browser: Google Chrome, Microsoft Edge or Opera, on Windows, macOS, Linux or ChromeOS. These support Web Serial, which is how the page talks to your device.
Safari and Firefox will not work. They do not support Web Serial, so they cannot connect to your hardware at all.
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.