Abnormal behaviour of MacBook 16" of 2019 after repair by local vendor

Fan and temperature sensor issues a easier spotted by Apple Diagnostics.

Use Apple Diagnostics to test your Mac - Apple Support

Apple Diagnostics reference codes - Apple Support

It also should be able to show battery issues.

In the past, some overheating issues reported here were due to techs failing to simply reconnect the power wires to the fans.

— Is it possible that the vendor might have changed or replaced some important component of MacBook like battery, fan exhaust etc.

It is very possible

— How can this be checked?

A diligent repair shop will provide printed documentation showing cost by parts and labor, and providing a list of parts.

Hi,

I'm not sure your local "vender" is Apple Authorized or not, though if battery was changed, you can see it at System Preferences > General > System report.

Did they also update MacOS?

A new version of MacOS works very hard doing indexing the first few days after install, and you should also connect your external backup drive and allow it to make a disk-based copy of your files with Time Machine.