There is a special device that hooks to the hot and cold taps at the farthest faucet. Inside that is a temperature-sensitive device that is open when the water on both sides of it are cold. On demand, or on a timer, the recirculating pump sends hot water through the hot water line, to this valve, and then forces it back on the cold water line. It does this until the temperature valve senses that hot water is now available at the faucet. At this point the valve closes, leaving hot water at the hot faucet.
There’s a lengthy writeup here:
The loop is completed by the temperature-sensitive valve at the farthest end from the heater. This goes under the sink at the faucet connections. You could probably do this as you don’t need into-the-concrete access.
Here’s the manufacturer page:
https://www.watts.com/products/plumbing-flow-control-solutions/water-heater-accessories/hot-water-recirculating-systems#