Check the motherboard manual or look for a setting in BIOS that turns off the CPU fan below a certain temperature.
In Gigabyte boards it is in the Smart Fan section. CPU Fan Stop enable/disable.
Similar to most modern graphics cards that support Zero RPM feature. The fans stop when they are not needed.
Ryzen R7 5700X | B550 Gaming X | 2x16GB G.Skill 3600 | Radeon RX 7900XT
Ryzen R7 5700G | B550 Gaming X | 2x8GB G.Skill 4000 | Radeon Vega 8 IGP
Ryzen R5 5600 | B550 Gaming Edge | 4x8GB G.Skill 3600 | Radeon RX 6800XT