AMA - Follow the prompt: How data flows through Microsoft 365 Copilot

recently had a client who wanted a m365 copilot license but when I asked him about his base m365 license he said he has business premium but he also has email accounts with m365 std licenses for 4 other domains(one tenant ) where he checks emails on his base license outlook app but he cant use one email address for all due to legal reasons as he has to join meetings with those  account. what is a good solution if any to ground all the data from email plus teams