Brother got scammed by fake check

So my brother was recently approached by a person online who wanted him to tutor his son maths online, everything went well until the payment came, they paid via check, they sent him the pictures of the check and asked to print it and mobile deposit it through the bank app, the check went through and funds were received, so the person who approached decided to cancel the agreement and no longer wanted the services and wanted my brother to refund him via cashapp, my brother did that and some days later the check bounced or something and now he's in negative $2k. The thing i want to understand is, how did the check went through in the first place? Are there no verification systems in place? Also who did the scammers even use the account details of on the check for it to go through initially.