Depends on the relaying and circuitry. If a backfeed is possible, the “old” way was to switch it manually. With newer technology, circuits can now be sectionalized and backfed automatically. This, of course, is much more expensive and complex, so it is often just used on lines with critical loads, like a hospital. And no, not all power circuits are closed loops. Many (most) transmission circuits are closed loop (69 Kv and above), but most distribution circuits (below 69 Kv) are radially fed.
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