https://wccftech.com/base-m1-imac-features-one-fan-cooling-system/

While Apple might not incorporate any changes to its products on the outside, the details on the inside are more pronounced. You may not have noticed this, but the base M1 iMac and the higher-tier options have several differences on the inside, subtly suggesting that if you want the best possible performance, you will have to spend more money. In short, the biggest difference is that the $1,299 24-inch iMac features only a single fan for cooling the M1.

YouTuber Luke Miani pointed out on Twitter that the base M1 iMac features just a single fan for cooling the chipset. If the thread is opened, we find out there are more changes than Apple is letting on. It appears that the least expensive option only features a heatsink to transfer the heat generated by the M1. If you decide to spend a little extra money and get the 8-core CPU and 8-core GPU options, you do not just get two fans for additional cooling, but one heat pipe to effectively transfer the heat away from the SoC.