Here's a picture: https://ibb.co/RgfZfPR
Firstly, they aren’t on the heat shield, but on the part of the capsule it shields.
https://en.m.wikipedia.org/wiki/Atmospheric_entry#Ablative explains how that shielding works:
”The ablative heat shield functions by lifting the hot shock layer gas away from the heat shield's outer wall (creating a cooler boundary layer)”
So, it’s not only that insulation layers in the heat shield slow down direct heat transfer to the capsule. Also, evaporation of the heat shield pushes the place where heat is produced away from the heat shield.
The shape of the capsule also helps, as it prevents (most of) the hot gases from coming close to the wall of the capsule. https://blog.scienceandindustrymuseum.org.uk/re-entry-gettin...:
“WHY IS THE CAPSULE BLUNT?
When we think of vehicles that are designed to go fast, they are usually sharp and slender to minimise the drag. However, with a space capsule we want to maximise drag to slow down for a safe landing. The blunt capsule design was first proposed by Harvey Julian Allen in 1953 at NASA Ames research centre. It had two advantages—not only the high drag needed to reduce speed but also the blunt shape, which resulted in the shock wave detaching from the surface of the vehicle. This helped to reduce the very high temperatures (around 1,648°C for the space shuttle nose) from directly contacting the surface, which would otherwise melt.“
(That page has a photo of the shock waves around a Soyuz during recently)
You can see it dropping off @ 5:20 here: https://www.youtube.com/watch?v=GWusG5EW3dU