Rav Eliezer Melamed in his comprehensive and well-organized Peninei Halakha (17:12) explains
From the beginning of Elokai netzor until the second time
Yih’yu l’ratzon is recited, one is permitted to respond to Kaddish,
Kedushah, Barchu, and Modim – like the law regarding one who is in the
middle of Birkot Keriat Shema – because the person praying has already
concluded the main part of the Amidah. Nevertheless, since he did not
entirely finish the prayer, he is prohibited from answering Amen after
berachot or anything similar.
For the second question:
Upon the conclusion of the prayers of supplication, the person praying
recites Yih’yu l’ratzon again and takes three steps backwards. If he
cannot step backwards because the person behind him is still praying,
even though he has not yet departed from the Amidah, he may respond
aloud to any matter of sanctity, even “Baruch Hu u’varuch Shemo,”
since he already finished the supplications after the Amidah.
Similarly, he is permitted to continue praying, reciting Ashrei and
Aleinu L’Shabe’ach, or he may recite Psalms or learn Torah (see
further in this chapter, halachah 20).