ATMega328P U Query - does it work straight away on Uno R3
The MCU on the Uno R3 comes with a specific bootloader (Optiboot) and
fuse configuration (external clock, clock prescaler disabled, bootloader
enabled...). You may find some ATmega328Ps on the market that have been
specifically configured as drop-in replacements for the one on an Uno.
Those should “just work”. If the ones you bought come with a different
bootloader or fuse configuration, you may have to re-flash or
reconfigure them.