Howe Do I Prioritize Which Mac (all On iCloud) Is the Primary Destination?

I have 4 Macs, one on Mojave, one on Yosemite, one on High Sierra, and one on Snow Leopard (different because of their ages and capabilities). All are on some version of iCloud. All on the same WiFi home router. I want to have all my saved and filed email on one Mac so I can file email on the sidebar. Problem is that I use all 4 and filing is Mac specific. I’d like to file it on one of the 4 and have it all end up on one Mac with Mojave. Possible?
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John Haller