What's the best alternative to Apple OS since my mac is disowned by the corporation? Not a fan of windows, but my memory, battery, and 1tb SSD are not that old, and I gave a good clean and new thermal paste for CPU not long ago.
MacBook, OS X 10.11
Linux Mint is a good option, but it will require you to learn a new OS. Linux is not for everyone, but Linux is a great way to extend the useful life of older computers. Linux can use many of the popular web browsers (Firefox, Google Chrome, Vivaldi, and others), plus the distribution's software repositories have access to lots of free open source software such as LibreOffice. Linux Mint is easy to install and should just work on a Mac. A co-worker installs Linux Mint on some of our older Macs to give out to those less fortunate. There are other Linux options, but not all of them will easily work on a Mac, nor do many of them work on systems with limited resources.
Kubuntu and Ubuntu-MATE are two other options which utilize different desktop environments using the Ubuntu core. I'm not a fan of Ubuntu, but it is usually easy to install & use as well, although Ubuntu has recently been making core changes which some people dislike. Linux Mint is based on Ubuntu, but with some significant changes (I'm not a fan of some of those Linux Mint changes either, but I think the Linux Mint team is trying to address some of them). This is definitely the way to go if you are new to Linux, but are also good choices for using Linux on a Mac if you already know Linux since Macs may need some extra custom configurations after the initial install for some Linux distributions). The default Ubuntu uses the GNOME desktop environment which tends to require a more powerful system. All of these distributions utilize the same core, but utilize different Desktop Environments (aka GUI interface) and different default apps.
Many times you can test out a Linux distribution by choosing the "Live USB" or "Live Mode" option when booting to the Linux installer. Keep in mind performance will be extremely slow when booting from a USB stick, but it can give you a general idea on how Linux will look and the default apps installed.
Use the downloaded Linux .iso file as a source for Etcher (Mac/Windows/Linux) to create the bootable Linux USB installer. Option Boot the Linux USB stick and select the orange icon labeled "EFI". Keep in mind macOS will not be able to mount the Linux USB stick within macOS and may not even recognize it as bootable in the Startup Disk System Preferences (all of this is normal), but the Apple boot picker menu when Option Booting should see the bootable Linux USB installer.
I highly recommend you create a bootable macOS 10.11 El Capitan USB installer now while you can still boot into macOS so if you change your mind later on, you can still reinstall macOS easily. Here is an Apple article with links to various macOS installers as well as instructions for creating a bootable macOS USB installer (read the instructions carefully as the macOS 10.11 installer has several required steps to extract the installer to the Applications folder):
How to create a bootable installer for macOS - Apple Support
Late 2008 MacBook OS
