Thank you. I had previously downloaded libraries from "near" those links, and found out that the "most recent" were 10 years old (specifically DS3231.h)... Is there a way I can know "most recent" is most recent? Thanks again.
It is not clear to me what you are asking. Are you interested in the most recent version of a library, or the most recent version of the Arduino IDE?
If you are downloading libraries from a downloads.arduino.cc URL, then they must be the Arduino Library Manager libraries. There is a JSON format index of them all here that could fairly easily be machine parsed:
arduino-cli lib install AudioZero # for the latest version.
As for the Arduino IDE, that is a bit more tricky. There is a discussion about it here:
There are some commands to get the version shared there. Make sure to read to post #6 for the corrected version of the git ls-remote command.
Those commands are parsing the list of tags in the Arduino IDE 1.x repository (https://github.com/arduino/Arduino.git). Arduino IDE 2.x has a new repository (https://github.com/arduino/arduino-ide.git), which is not discussed in that thread because Arduino IDE 2.x did not exist at that time. But the same commands will work just as well for Arduino IDE 2.x if you just change the repository URL in the git remote command.
# Arduino IDE tag blocklist:
# <1.5.2: no CLI (https://github.com/arduino/Arduino/blob/master/build/shared/manpage.adoc#history)
# 1.5.4-r2: Not available for download
# 1.5.5-r2: Not available for download
# 1.5.7-macosx-java7: Not available for download
# 1.5.8-macosx-java7: Not available for download
# 1.6.2: has the nasty behavior of moving the included hardware cores to the .arduino15 folder, causing those versions to be used for all builds after Arduino IDE 1.6.2 is used. For that reason, 1.6.2 will only be installed if explicitly specified in the install_ide version arguments
# 1.6.5-r2: Not available for download
# 1.6.5-r3: Not available for download
# 1.6.5-r2: Not available for download
# 1.6.5-r3: Not available for download
# 1.8.11-ms-store-1: Not available for download
# 1.8.13-ms-store-1: Not available for download
local -r ARDUINO_CI_SCRIPT_FULL_IDE_VERSION_LIST_ARRAY="${ARDUINO_CI_SCRIPT_IDE_VERSION_LIST_ARRAY_DECLARATION}'(\"$(git ls-remote --quiet --tags --refs | grep --invert-match --regexp='refs/tags/1\.0' --regexp='refs/tags/1\.5$' --regexp='refs/tags/1\.5\.1$' --regexp='refs/tags/1\.5\.4-r2$' --regexp='refs/tags/1\.5\.5-r2$' --regexp='refs/tags/1\.5\.7-macosx-java7$' --regexp='refs/tags/1\.5\.8-macosx-java7$' ${IDEversion162regex} --regexp='refs/tags/1\.6\.5-r2$' --regexp='refs/tags/1\.6\.5-r3$' --regexp='refs/tags/.*-ms-store.*$' | grep --regexp='refs/tags/[0-9]\+\.[0-9]\+\.[0-9]\+\(\(-.*$\)\|$\)' | cut --delimiter='/' --fields=3 | sort --version-sort | sed ':a;N;$!ba;s/\n/\" \"/g')\")'"