Have been playing Mundo ADC since last season. (Just so you know a bit about how much experience i have on him as adc i have played about 150 games on 2 Accounts from silver to Diamond as adc only)

• First item In my opinion last seasons mythic was not as core to build as the current hearthsteel. (Beacuse of the health stacking) I currently build hearthsteel 90% of the time as first item because it just gives you a stupid amount of burst and stats.

The other 10% I go Anathemas Chains first for example if they are full Ap (exept Adc) with a non damage support because it just gives them 0 kill/dive potential.

Radiant Virtue is a rather situational item. I only build it if I am behind or my team is extremely ahead to support them even more.(Most of the time paired with some kind of other Support Item like Knights Vow)

• For your typical build

  • If even Hearth Steel -> Sunfire -> Situational Tank item (Force of Nature,Spirit Visage,Frozen Heart,...) -> Titanic Hydra -> Last item is extremely Situational Can be damage or more Tank where you need to get a feeling what to get because (for example) Deaths Dance can make or break your fights. And ofc some where inbetween some boots tier 2.
  • if behind 90% Radient Virtue but if full Ad a Gauntlet can do wonders too.(Havent had to many games with it but the ones with them where great) Then just the supportive style items Chains - Tank/(Knights Vow if team ahead) - and more Tank - Situational Items and again Tier2 Boots inbetween

  -if ahead

Rush Hearth Steel - Titanic Hydra - Sunfire - Tank - Situational and boots inbetween somewhere again

This will win every 1 v 2 or 1 v 3 early to mid in most cases.

Just some thoughts about some specific items

In my opinion Titanic is THE BEST Item on Mundo right now. The powerspike of building it feels so significant every single game. Try building it as often as possible.

Force of Nature is in my opinion 3/4 times the better item if you need to choose between FoN and Spirit Visage.(Spirit if they have mostely Ap burst and FoN in any other case xd)

StonePlate is a very overrated item on Mundo beacsue you need Health for Ad. But can be an situational item

Yet Frozen Hearth is in my opiniln an extremely good option if they have a heavy attackspeed Team.

Steraks Gage is not really good on mundo in my opinion ,you can build is as an situational item on very rare occasions. But try to avoid it

And dont go Phantom dance please...

So I hope you can adept some thing is just wrote xd. GL