Think of ZeroTier network as a generic router.
What you have is something like this:
LAN1–R1—R3–R2–LAN2 or LAN1–R1—R3–PC
R1 (and R2) is your router. R3 is Zerotier.
“Managed Routes” is equivalent of the static route configured on R3:
LAN1 (192.168.10.0/24) is reachable via “R1 ZT address”
LAN2 (…) is reachable via “R2 ZT address”
Once configured these routes will be automatically propagated to all the ZT network members like routers and standalone PCs.