When do the Pokémon from HeartGold evolve?

Answer 1

It depends which Pokemon you mean.

Some like Pidgey, Totodile, Chikorita, Cyndaquil and others evolve by leveling up.

Some like Celebi, Pachirisu, ONOWN, Groudon, Aerodactyl and others don't evolve at all.

Some like Eevee, Misdreavus, Murkrow, Vulpix and others evolve by using an item on it.

Some like Buneary, Togepi, Riolu, Golbat and others evolve when you make it happy.

Some like Kadabra, Machoke, Graveler, Haunter and others evolve when they are traded.

Some like Rhydon, Seadra, Electabuzz, Magmar and others evolve when traded with an item attached.

Some like Burmy, Combee, Kirlia and Snorunt evolve depending on their gender.

Some like Gligar, Sneasel and Happiny evolve when leveled up with an item.

Some like Eevee, Magneton and Nosepass evolve when leveled up at a certain place.

Some like Lickitung, Yanma, Piloswine, Bonsly and more evolve when they learn a certain move.

Mantyke evolves if a Remoraid is in your party.

Feebas evolves when you max out it's beauty.