What is the thing that is placed inside a traditional buddha?

The Tibetans use/write/carve mantras in many things they create.

looking at some statues , i would say merlin car tyres.

This is all I could find
Hope it may help
Buddha Statue Amulets.
These small Buddha statue amulets were originally made to be buried beneath temples and many have been found inside much larger Buddha statues. The reason they were often placed inside other much larger Buddha statues is they were thought of as ‘Storage batteries of Buddhist energy’. A single small Buddha or a few small Buddha’s inside a large statue gave the statue more powerful energy. Small Buddha statue, amulets were also made for domestic worship in the home and were also carried for protection. Small Buddha statue, amulets were and still are left at temples as votive offerings. Most of these small sacred Buddha statues are made from solid bronze (It is known that old Tibetan and Nepalese ‘singing bowls’ were often melted down to make these small Buddha statue amulets.) Bronze is not an easy metal to work with and the modelling of many of these small Buddha statues is often very eccentric, and it is not always easy to link the statue to any of the known major historical styles of Buddha images. The fact that so many of these Buddha statue amulets were made over the centuries is a tribute to the piety of the Buddhist artisans who made them.