I have tMobil/sprint for a carrier. They send me SIM card to replace an existing one so that I can get a better reception. Is that true?

The SIM is a subscriber identification module, and is not involved in radio communications.

Indirectly, switching a SIM card can cause the iPhone to access different carrier network services within the carrier networks, or to be authorized by the carrier to access different parts of the carrier network, and some of which can quite possibly involve better reception or faster speeds or other benefits.

Various folks with newer iPhone models have needed to swap to a SIM that allows access to 5G networking, for instance.

As guess, this probably is a SIM that will allow an iPhone 12 or newer access to 5G networking, too. Which also opens up some useful features on the iPhone, such as iCloud backups via cellular, and iOS updates via cellular. This with most 5G carriers and with a 5G connection available.

So is this “better reception” true? Probably yes. Particularly if there is a 5G network built-out and with better coverage in your area, and you presently have a SIM for LTE/4G.

The Sprint LTE network went dark on June 30th as T-Mobile turned it off. They reportedly were sending all of their Sprint customers updated SIM cards to better connect on the now solely T-Mobile LTE/5G network. I saw a couple of tech news items about it a few months ago.

https://www.t-mobile.com/support/coverage/t-mobile-network-evolution

My sim was very old. So a while ago, my carrier sent me a letter which stated that my old sim was not good enough anymore, and that I get a new one. This was entirely true. I still had a 3G simcard and my carrier was dropping the 3G network