https://fanlesstech.com reviews lots of completely silent computers generally using standard components. I would go with something with standardized components if you can as the case, the power, the heatsink etc may all be reusable.

https://simplynuc.com will equip you with a NUC mounted in a fanless case. This means a proprietary motherboard/soldered chip and a case no other computer can be mounted in, which left me with a rather expensive and useless case when an older NUC died. A NUC is a very small line of computers made by Intel, they're basically laptop hardware repurposed as desktops.

https://quietpc.com has lots of fanless and near-silent machines too up to full workstation-size desktops but the smallest ones all tend to be the Intel NUCs.

I currently have a 6-core/12-thread NUC with a 1tb NVMe which is even smaller than a Mac Mini. But I hesitate to recommend this as it has a 10th generation processor saddled with the years-old Intel HD620 graphics like the Mac Mini, my 2013 MBP with Intel Iris outperforms it. It requires an eGPU for anything even slightly graphically intensive.

https://liliputing.com/2020/04/intel-frost-canyon-nuc-10-rev...