At Bikable, we have a really large selection of different luggage carriers. Right for the very small bikes of 14, 16, 18 and 20", to junior bikes of 24 and 26" and for adult bikes of 28", and of course luggage carriers for mountain bikes, as well as for racing bikes too.
Traditionally, there are 4 holes in the frame with threads intended for mounting a luggage carrier
For these holes, use a classic luggage carrier such as this luggage carrier 26-28" in aluminium matte black
On some bicycles, however, there are disc brakes, here it is recommended to use a model that is specially adapted for these
An example of such a luggage carrier is this model; luggage carrier 26-28" in aluminium matte black disc brake model
But if you don't have the 4 holes for mounting a rear carrier, don't worry, because there are also luggage carriers for mounting on the seat post, like this Atran Velo Luggage carrier for seat post-AVS
In addition to the three examples above, there are also front-mounted luggage carriers, luggage carriers for mounting in quick release, luggage carriers with baskets, and luggage carriers for electric bikes
So there is plenty of opportunity to get a luggage carrier on the bike, and when you have the luggage carrier fitted, baskets, boxes, bags etc. can be fitted.