Install specific versions of AOSP Calendar I would like to use an older version of AOSP Calendar. Namely a good version before the 2019 update that is described here as follows:

New events cannot be created and existing events cannot be edited or deleted.

There are many branches and tags here, but some of their titles are not clear, and I don't see APK files there. How exactly can I install specific versions?
The AOSP Calendar is in a repository tool (Google Git) that use Google with a lot of project.
Compile it by yourself
You could download it:
git clone https://android.googlesource.com/platform/packages/apps/Calendar

and you  could select the branch/tag that you want.
git fetch origin nameBranch/nameTag &amp;&amp; git reset --hard FETCH_HEAD &amp;&amp; git checkout nameBranch/nameTag

Of this way, you obtain the code of the Calendar AOSP but as you said, in this repository is not the package of the application, so, you should compile it. As it is part of the Android system is neccesary to compile it with the rest of AOSP, because, it has some dependences.
LOCAL_STATIC_JAVA_LIBRARIES := \
    android-common \
    libchips \
    colorpicker \
    android-opt-timezonepicker \
    androidx.legacy_legacy-support-v4 \
    calendar-common

Once you had it compiled, you don't have problems to install it, because althought it is a part of AOSP it is not a system application.
But, this is not a quickly way to obatin the APK file.
Obtain it of system.img
First, you need to obtain a AOSP or Lineage ROM system image before the 2019 (because, the commit that you don't want is in 2019). For this way, you need to use this tool. Once downloaded and installed:
mkdir system_lineage
simg2img system_lineage.img system_lineage.raw
sudo mount -t ext4 -o loop system_lineage.raw system_lineage
cd system_lineage

And you need to found it in this system directory (I think that it was in system/app/Calendar or something similar)
Try to obtain it in some mirror
It is possible that some mirror have this apk in the server. You could try to obtain it in apkmirror for example.