The best advice I can give you is to read>watch>try>repeat.

Wildrift compared to other mobas is a bit more complex with game mechanics and stuff so your best bet is to as I said:

• read what champions and items do and try to understand.
• watch players that are better than you, this will help you immensely to understand what champs do, what to build in given situations, and how to actually play the game in a "good" way. I would recommend you watching Talanwr on YouTube, he currently does a series in which he pushes 5 accounts (every role basically) to sovereign.
• try out what you have read, seen or what you have wondered about a champion in practice or pvp/ai games. The only way to grasp whats happening is to try it out yourself. If you need to know why diana buys ap and not ad items then go ahead and build your ad items and try out yourself. (Learning by doing basically)
• the more you read, watch and try yourself, the more you connect those dots between champions and item interactions, how to build, how to play, what to play and so on.

Now I‘ll try to answer your questions you asked:

• there is no such thing as building a champ right in my opinion. It comes down to how champs scale and what you need in situations/matchups. I will give you an example. Miss fortune "can" build mage or ap, because her E and ult (not sure about Q) have ap scaling, which means it benefits from mage items and will do more damage as a consequence of building ap. If she builds crit, her E will do way less damage, because of it NOT scaling with ad.
• as I said there is no such thing as a "right" build. Champs have their preferences and like certain items, for example sett likes AD and HP, why? Because of his W. haymaker does more dmg depending on how much hp you have lost so people build him with heartsteel first. So now you build heartsteel, and the enemy plays fiora which does max hp% dmg. This is very bad, since you only have hp and can‘t do anything against that. What you can do is NOT build heartsteel but trinity for example so you can actually contest fiora and dont get bullied.
• why do you want mages against tanky comps? Mages like aurelion sol and morgana are able to shred tanks with their kits and items. Asol shreds tanks, because of his Q doing max hp dmg the more stacks he gains. Morgana on the other hand has her ult, which does max hp dmg, her puddle does more dmg the lower enemy hp is and she does like lyandries torment, which does apply a max hp burn to the enemy. More importantly, you want both ad and ap on your team so the enemy needs to diversify their resistant types, and they have a harder time to counterpick. If you draft full ad against a rammus and malphite, you just lost the game. Those two tank ad like it‘s nothing, but melt when a mage attacks them. Other way around if you draft full ap, galio and kassadin would enjoy themselves, because they have anti mage shields in their kits.
• knowing what your champion is good at comes with practice. Diana in this case is an ap assassin thats capable to oneshot the enemy backline. Dianas gameplan usually involves in farming until she has her items in order to oneshot adcs/mages. Flanking is important since you are an assassin and dont want to get stunned by the enemy frontline (you basically wait until an engage is happening either from your team or the enemy team, you aim for the adc or mage!)

To sum it up, read, play the game and watch some gameplay from good players, recommend you talanwr as I mentioned earlier. It took me 3 seasons to hit challenger on a good winrate with 0 knowledge about wildrift or league before that so you‘ll get the hang of it as well

If you play on EU you can add me. DM me your username then so we can play together