The city is called Phoenix, Arizona. These are overlapping political jurisdictions; Phoenix a subunit of Arizona, Arizona itself a subunit of the USA, the latter an abbreviation - humans communicate by making sounds with fibers in their throat, which is slow, so they have abbreviation as a standard feature of communication - for the United States of America, a sort of governing entity that stretches across this continent. That much they can determine without talking to any humans. For more than that - for all the details they will need to win this war - they will have to talk to humans, and so they will have to abduct some.
They are excruciatingly careful not to pick up any Controllers, or anyone who has people in their household who are Controllers, and they try not to pick up anybody who will swiftly be missed. They get two dozen; enough everyone can acquire human morphs that express the full range of human phenotypes, enough that it is probable that at least one or two will be cooperative, not enough that they are running much significant risk of exposure, and not enough that if this turns out disastrous and they have to quietly kill them all it will be immediately obvious that there were Andalites present here.
He cannot imagine what would transpire that would make that necessary, but this war effort has been a series of catastrophes and failures of contingency plans and failures to make contingency plans, so he has prepared for the possibility he needs to execute a few dozen innocent people. He's dreading it, and rather dwelling on it, despite assigning its necessity a very low probability. (They have already checked whether memory drugs work on humans; they do. What else could go wrong? But he'd asked himself that question before to be unpleasantly surprised...)
He's watched them for three days, he is fond of them all. But there are six billion people on this world he would like just as well, if he'd had the chance to meet them.
Humans bear live young, and he has one young human here, whose parents were intoxicated and spent the necessary three days of observation mostly injecting substances into their bloodstream and yelling at the young human. He should be able to return all three of those home without incident, or replace them without arousing suspicion. He also has two elderly humans who did not get any visitors over the observation period and similarly won't be missed.
The rest they can't keep for more than the eight-hour human dormancy period without arousing suspicion: eight male and seven female human adults, some coupled and some living alone, all in the same apartment complex that can at need catch fire, two adolescent human boys and two adolescent human girls who travel to the human academy in vehicles that crash not infrequently.
Humans don't vary much in phenotype; they can change out their outer skins for ones in a bewildering array of colors, and their inner skins come in varying shades, all of which are represented in his sample, but they all have two legs and two eyes and the same approximate face shape. He samples them and tries a few different mixes and arrives at a human phenotype he finds satisfactory. He morphs. He practices walking around on two feet. He practices making facial expressions.
And he waits for them to wake up.