Yeah, what you said is the absolute truth, managing and or coaching a national team is completely different and way more difficult than coaching a club, and the reason you gave is very valid, finding good players for a club is easy because there is no such bound or limitedness in the sense that a player from Nigeria( for example) can not play for Brazil and a Brazilian player as well can not play for the Nigerian super eagles, and it is exactly like this for every other countries as well, so theres is a limit and every country has to make do with their own players and not get a player elsewhere outside the country.
And speaking of Brazil in particular now, the coach will simply learn to deal personally with all the players, learn them and now how to treat each one of the based on their individual attitudes, this is one way of becoming a good and successful coach.
Let's say that there are regions and countries where it is easier to select players for your national team, like Brazil, which doesn't have any difficulty selecting players but rather has difficulty doing so because there are too many of them, while Arab teams have a hard time because nobody plays football and they have to force the players to naturalise.