You can download the latest Magisk app from GitHub here: https://github.com/topjohnwu/Magisk/releases. SuperSU was taken down from the Play Store years ago and Magisk is not allowed on the Play Store either.
Once you open the Magisk app, you want to patch a boot image. This is done by downloading the firmware for your smartphone, extracting it, and then checking in your file explorer for the boot.img folder containing the boot image.
Note that some smartphones use the A-only partition scheme. If yours is one of them, you can find the boot image (boot.img) file inside the recovery-flashable ZIP file.
On the other hand, if your smartphone has the A/B partition scheme, the boot image (boot.img) is found in the file named payload.bin.
Where you get the firmware for your smartphone will depend on what manufacturer you use. Many manufacturers provide firmware files; however, some of the largest brands do not. Samsung is one brand that does not provide firmware; however, there are tools that allow you to download the Samsung firmware directly from Samsung’s servers. Tools such as SamFirm_Reborn Tool, Bifrost – Samsung Firmware Downloader Tool, or Frija let you download Samsung firmware files to your computer so you can extract them and locate the boot.img files.