The shares that are 'borrowed" are from insurance and...

The shares that are 'borrowed" are from insurance and superannuation companys.
The shares "borrowed" are third party shares. Lets say a superannuation company be it industry or ones that a busy person may pay into.
I will use an industry superannuation fund as an example.
They can only purchase within the rules of that fund, maybe no purchases below the ASX 500. It is at the discression of the fund manager how he spreads the risk. That same fund may allow members to choose individual stocks they want the fund to purchase. They may also allow ASX300 stocks for the member to choose from.
Now any shares purchased are in the name of the insurance/superannuation fund and are pooled.
They are not purchased in the name of the individual member.
They are known as third party shares. These are the shares that are "borrowed". If you do not have a CHESS or Issuer Sponsored Certificate, they can be "lent" .
The fund owns them.

Now shares that are held in SMSF (Self Managed Super Funds) are held by the owner of the shares and cannot be borrowed without the owners permission. The owner has a CHESS or Issuer Sponsored Certificate of ownership.

This is not financial advise please seek profesional advise in this matter.