TheFragilemustardoverlordfragile gave really good advice, but I feel there's one thing that needs to be added:in the modern hl "metagame", the majority of the top teams have shifted away from a super heavy-centric style to a more demo-centric one, with more heals towards the scout and pyro as well. the heavy is still the best class at protecting the medic, but his speed means it's really hard to use him aggressively in ubers, so in more aggressive situations medics typically lead with demos (and sometimes scouts or even pyros depending on the situation) and use the heavy as kind of an anchor to protect him post-ubertl;dr mad healz to the demo
I would imagine heavy and demoman stick together, so the demoman can push with heavy support and then the demoman can fall back on the heavy when needed. That's ridiculously good, haha. But yeah, I have played hl only a few times, so I'm sure mustard is right.
yeah, that's the general idea
if you look at heal stats nowadays, demos usually trump heavies, but you also have to factor in the fact that those stats don't include buffs and the way heavies play often prevents them from falling as far below full health as other classes
it's definitely still true that there are occasions where heavies lead pushes, like in a lot of situations where you just need to use that wall of health (e.g. him being the first guy to walk onto the point on a koth map repush), it's just that like a year ago teams seemed to ALWAYS have the heavy lead pushes, which has changed a lot
hell, -ts- (the team that won last season) had ruwin on scout, and they often used him extremely aggressively in uber fights to chase the enemy if they had a worse uber
EDIT: it's a lot like 6s where the heavy takes the place of a pocket soldier, but while the pocket has the edge in both protecting the medic and using ubers while the demo just has the edge in damage, the heavy isn't always super great during uber fights, so rather than demos taking shelter behind their pocket they lead a lot more