How do I transfer GEDCOM file from Ancestry to iMac?

First, export your tree from Ancestry:

• Log into your Ancestry account
• Open your tree
• Go to Tree Settings
• Under Manage Your Tree click Export tree
• Ancestry will create a GEDCOM file. When done, there will be a blue button that says "Download your GEDCOM file"
• Click the button and it will download the GEDCOM file to your iMac.

On your iMac, open whatever genealogy program you use and follow its instructions to import the GEDCOM file.

For example, I use Reunion for Mac (which I think is the best geneaolgy app for Mac users).

In Reunion,

• Create a new family tree (a blank tree)
• Click File > Import > GEDCOM
• The GEDCOM will be imported into Reunion.

Most genealogy apps and genealogy websites can import and export GEDCOM files.

(There can be wrinkles too, as there are differing interpretations of GEDCOM among different apps.)

The Gramps app is one of many examples of available genealogy apps.

For assistance with a specific third-party app and its features, check with the app vendor support, or ask in a forum that discusses the particular app of interest.