gTLD records from rootDNS process question

Ultimately it depends on your client, but typical client behaviour is to take them in the order received - that is, try the first one, if that doesn't work, try the second, if that doesn't work, try the third etc.

Some DNS servers will randomise the order that they return records in, in order to more evenly balance traffic. GTM goes a step further than that, and returns the 'best' response (where "best" is determined by the GTM's configuration)

With regard to your second question, where the delegated DNS server returns two NS records pointing your GTMs, it doesn't actually matter which GTM they hit, because all GTMS in the same sync group will return the same answer, given the same question from the same LDNS IP address.