Let me see if I can answer the questions you've asked:
How does the agent decides which file to backup
Any file modified by a user gets backed up.
You can exclude certain directories if you prefer.
Are the files backed up when accessed by a process (does that mean the process have to wait the backup is completed before running) or is the agent actively looking for these files (and if so how - only local or also remote on file servers?)
As stated above, files that get modified get backed up.
I believe that also includes remote fileshares as well.
- How long is each file kept in the backup
- I can configure a backup size limit in the Endpoint Management but what happens when the client reaches the limit (I assume some files will be deleted, but which one)
Think of it as a first in, first-out buffer.
I see in the documentation that the agent is supposed to create some random files in My Documents, etc. I cannot find these files however I see folders such as "CheckPoint!FrameworkDirectoryDon'tDiscard" and "Sandblast Zero-Day-SystemFolder-Do notDiscard" but again no information on what they are
I assume these exist for similar reasons to the ones documented above, but will admit I don't know exactly what these folders are for.
How is the anti-ransomware agent "constantly monitoring suspicious activities", I understand for this one that details might be restricted to not have other vendors copying it but at least some high level description would be useful
Basically anything that would be inconsistent with normal user activity is flagged.
Modifying a large number of files at once is certainly suspicious, as is modifying our random files.