What are the similarities and difference between the elements in an early protostar and those in a young star formed from the stardust of older stars?
Early protostars and young stars will have slightly different ratios of elements.
Both early protostars and young stars are formed from a clod of gas which collapses under gravity to form a star. Both types of star are mainly Hydrogen and some Helium.
Early protostars would have been formed from the gasses that were created soon after the big bang. They would be 75% Hydrogen, 25% Helium with traces of Lithium.
Young stars formed out of the remains of old stars would still be mainly Hydrogen. They would also have have small quantities of heavier elements which were formed by fusion reactions in the old stars. They would contain some carbon, oxygen and traces of heavier elements.