In Baltimore, Amos has a lifetime worth of criminal connections. When he returned to Earth in Nemesis Games, he quickly tracked down his old associates, got the information he wanted about the woman who raised him, learned who was in charge and how that could benefit him - basically, he was able to leverage all his old connections almost as if he'd never left.
Alex doubts that Amos can work any of his usual underworld schemes on the occupied Medina Station - not only is it controlled by a hostile military, but most of the population aren't even Earthers. Amos barely knows anyone there, and he's an Earther himself, mistrusted by Belters. Not only would he be starting from scratch in forming new underworld connections, but his obvious Earther appearance would put him at a disadvantage. When Alex says "It's not Baltimore", he's pointing out that Amos doesn't have preexisting connections or an influential reputation on Medina, and he can't act like he would in Baltimore, where he has those things.
When Amos says "Everywhere's Baltimore", he's asserting that he's just as much at home in the illegal underworld here as he would be back in Baltimore, or in any city, on any station. Everywhere has criminal communities, and black markets, and political discontents, and a disenfranchised underclass - and Amos knows how those parts of society work. He's absolutely confident that he CAN build those connections from scratch, even in that situation, with his understanding of that type of people and the social and political structures they are forced to live under. And he turns out to be right - it's not long before he connects the Roci crew with political activists who are plotting against the Laconian occupation. He accomplishes it just by working alongside those who he sees as his type of people, talking to them, being part of their community. The same ways he built up his place in Baltimore's underworld work anywhere, as long as he can spot the right people.