Explain the concept of a risk-free rate. Is there a universal measure or proxy for it, or does it need to be regional?

Does a 10year US Govt. Bond have the same risk’as a 90day US TBill?

Explain the concept of a riskfree rate. Is there a universal measure or
prox
y for it, or does it need to be regional?

What is an acceptable maturity for it, or does it depend on its application