Difference between On-die and junction temperature
On-die temperature and junction temperatures are similar, the difference being:
On die temperature measurement is possible only if the device has a specific temperature diode sensor. On-die temperature is typically used by end use customers for various purposes - auto shutdown / power management / etc.
Junction temperature, on the other hand, is an approximation of the temperature based on calculation. whereas junction temperature is typically used by the chip manufacturer during characterization or manufacturing flow  or similar purposes.
Here is an on die temp measurement circuit for Intel Agilex

https://www.intel.com/content/dam/www/programmable/us/en/pdfs/literature/hb/agilex/ug-ag-pwr.pdf.