As with any 2 program running on any operating system, your ability to save a file in the first program and successfully use it from another depends on observing where the first program saved the file and being able to instruct the second program to read from that location.
Often browsers default to saving files in a "Downloads" folder. This may or may not be the case with your setup. If you're unsure, and you don't know enough to tell, please understand we can only really help if you share a screenshot so we can see the same pixels your eyes saw. if you don't yet know enough about Linux to make a screenshot and post it here on this forum, perhaps you could resort to using your cell phone or camera to take a photo of the screen, and then share that image using tools you know how to use.
Once you know where the downloaded file was saved, then in Terminal you would type a "cd" command to change to that directory. It might be as simple as "cd Downloads". Again, if you need specific help, remember we can't see your screen unless you show us a picture.
In terminal, you can type "ls" or "ls -l" to see a list of the files in the current directory. If you typed the "cd" command correctly, you should see the TeensyduinoInstall.linuxarm and TeensyduinoInstall.linuxaarch64 files.
Then you would type the chmod 755 command, and then the command to run the file. They're shown on the download page. Just be sure to change the "linux64" part to either "linuxarm" or "linuxaarch64".
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Since you downloaded both, simply try both of them. One will give you some sort of error about unknown format or file not found. The other should open the Teensyduino installer window.
One important point you should understand is the words "Note: Arduino from Linux distro packages is not supported". The installer only works on copied of Arduino IDE downloaded directly from Arduino's website. If you installed Arduino IDE using packages provided by Raspbian or Debian, it will not work. You will reach a point where the "Next" button will not enable. The installer checks if a valid-looking copy of Arduino IDE is selected. This too requires the skill of downloading a file in one program, then being able to open in with 2 others. First you will need to extract the archive, and then you'll need to select that location from the installer. Again, if you can't do this, remember we can only really guide your hands if you show us screenshots. If we can't see your screen, can't see what you actually tried and what actually happened, helping effectively is very difficult.