What is Android?
Android OS is an OS developed and controlled by Google based on Linux for portable devices. The OS and third-party installation files are closed and not accessible by the user. Configuration and other user data files are usually stored in built-in SD storage (/storage/extSdCard). Depending on the app, user data might use external storage (microSD, /storage/sdcard0).
One change that users can do is moving programs from their default location to another device storage (/storage/extSdCard) to external (microSD, up to 32 GB, /storage/sdcard0) or vice versa. This is very handy if you want to save data created by certain applications like how many levels you have achieved in Angry Birds (2009, Rovio Entertainment). Since most Android OS devices can be mounted as external storage device, a mobile phone especially or other device can be used as a portable HDD.
Some systems do not let users access the device storage (/storage/extSdCard) and do not have external (microSD, /storage/sdcard0). Applications like AirDroid allow users to access the device storage (/storage/extSdCard) via an intranet-like connection on port 8888 (for example, 127.0.x.x:8888 or 192.168.x.x:8888) of the device using a web browser.
There is no OS installation other than rooting. The most you can do (legally) is installing programs (.APK files). Depending on the device manufacturer and model, these .APKs files are available at Google Play, Amazon Appstore or other providers especially for devices marketed for children like Kurio (security enabled by CronLab) and Nabi. Note that there are providers with dubious reputations, which may include malware.
Since Android OS is open source, various vendors have forked the OS to support their proprietary hardware and/or proprietary formats. For example, Fire OS is developed by Amazon for its newer Kindle devices (since 2007) as well as the Fire TV microconsole for streaming media and games (April 2014) and Fire Phone (June 2014 to October 2015).
My experience so far has been as an user although I have tried hacking it without rooting it or breaking any legality or EULA. Since I have always wanted to learn the inner functions of a system or another, I am currently learning how to code my own apps for Android OS focusing on the possible distribution through the two main Android OS outlets Google Play and Amazon Appstore. Comparing it with other portable devices, I find it much more user-friendly. I can easily configure the device faster than iOS, for example.
Android OS has poor memory management, but much better than others. It does not release programs from RAM, which often causes latency. It is up to the user to release them manually by holding the Home key for a second or two and swiping out each program. Third-party programs like Clean Master by Cheetah Mobile can release programs from RAM automatically, as per the user's setting. From experience, I have noticed that Android OS needs at least 20% of device storage to run smoothly, almost like a /swap partition or PAGEFILE.SYS files.