"1 - When I create the VM shows one (default policy, but does not show the name of the policy). Which of my policies does he refer to as a standard among all of the ones we have?"
The default Storage Policy(SP) for a vsanDatastore is 'vSAN Default Storage Policy' by default but can be changed to any other SP:
"2 - I believe that each of these policies has a purpose, but is there an order as they apply? Or do they work independently? Do they work together?"
Each Object (e.g. a vmdk, vswp or VM namespace folder) can have only 1 SP applied to it - nothing else would make much sense as you can't have something that is simultaneously FTT-2,RAID1,OSR=100 and also FTT-1,RAID5/6,OSR=0.
"3 - These policies are applied during the creation of the virtual machines only? Is there any other option to apply them?"
You can select the SP applied to VM/Object at creation and re-apply any other SP to it any time (provided you have adequate space/resources/licensing etc. for the attributes of the Object with the new SP).
"4 - It does exist, but when I look at VMs and show nothing, does it mean that it is not being used?"
Yes, that tab shows the VMs that have components with that SP currently applied to them - if none have it applied it will be blank.
The non-vSAN auto-created SPs you see are expected and shouldn't really have much bearing on vSAN as they are not used for this - the 2 'VXRAIL' named SPs are specific to VxRail and VxRail support will likely say to leave alone (used for vCenter and VxRail Manager etc. and are OSR=100 so these don't run into trouble if you run out of space).