Cams break in within the 1st few seconds/minutes as mentioned above, fire up and take rpms up no load for a couple minutes...this also beds the rings, generally one will hear the rings bed with a slight increase in rpms a a distinct change in tone.
Ring beding is dependant on the correct patten and hone grade of the final hone on the bores.
In the production factory this is automated, if done by hand in a machine shop, usually and experianced operator does this job when sizing to each piston.
An engine that doesnt bed the rings may smoke a little and use oil...usually they will bed over the next 500 to 1000 miles...if still doesnt bed in...take to a remote car park and start burning rubber off the rearwheels.
New cars out of the factory, for many yrs, are 'run in' at the factory and bedding is checked as part of the QC so it is a non issue.
An early oil change, under 1k miles is a good way to see what's going on.
worth doing even on a new car, for a rebuilt engine within 10 miles or even after breaking in and tune...this flushes any debis or gasket sealer that maybe floating around, the oil and filter is inpected for anything that maybe a miss
Also, when rebuilding an engine
never reuse an oil pump..replace no matter how old
never reuse a water pump, unless near new
never reuse water hoses, espec the bottom radiator hose unless new new
Never reuse the radiator unless the headers are pulled and cores MANUALLY cleaned and then presure tested, no matter how new.
Older radiators where over built, newer model cars have far less tollerance...a flush is not effective enough to clear all blocked cores or partly bloked ones, any gasket sealant breaking off can block up partly flowing cores A common issue in engine failure.