You put the Mount on the rack that lets you fit a removable tail light on the clip.

then you take the light off and plug in the recharge cable ..

By now several companies compete for your money on those , (i dont keep track)
looks like asking amazon will have a product to sell you .. google will offer a lot of choices
Touring, in the Summer, I Rarely ride after dark .

Now that LED hub dynamo powered lights work so well , I Use them most (exceptions older bikes I dont want to buy//build another wheel for..They are seldom ridden)