Authorized apple dealers?

Apple' have published instrctions on how ot install RAM ina 2020 iMac:

Install memory in an iMac - Apple Support

Use on Crucial or Other World Computeirn (OSWC) RAM for the best results.

Apple has never installed additional RAM modules after purchase. On a 27", it is really easy to do it yourself or you can ask an authorized service provider.

Here is an install video. The vendor is recommended and sells the modules

https://eshop.macsales.com/installvideos/imac-27-inch-5k-late-2015-memory/iMac20-1/