As is often the case, people mis-state things.
There is no AK "lower". The AK has only a receiver.
Rifles like the AR-15 have an upper and lower receiver.
Most AK-47 and AK-74 parts are interchangeable.
Some that are not are:
Bolt carrier is specific for the -47 or -74.
Gas piston, is usually slightly longer in the -74 but many are used in either rifle.
Most other parts "can" be interchanged although many -74 parts were redesigned to be lighter and are different. However, they can usually be used in both rifles.
Triggers are the same with the only difference being single or double hook triggers. Some receivers are made to use both, and a single hook trigger type receiver can be easily modified to use a double hook trigger, and a single hook trigger can be used in a double hook receiver.