I assume you mean "add-ons"? All add-ons have to be built (compiled) specifically for each version of AC, as they utilize a variety of common information. So, you cannot simply copy an add-on from AC 20 to AC 21 and expect it to run. The developer would have to recompile it for AC 21.
One of the forum moderators • AC 25 USA and earlier • MacOS 11.6.6, iMac Pro