Short answers to the questions the game never explains. Every number here is read out of the game's own data files, so the guides update themselves when the game does.

• Pokémon GO Type ChartEvery multiplier in one grid: 1.6x super effective, 0.625x resisted, no immunities.
• What Is Mega Evolution in Pokémon GO?What a Mega changes, which Pokémon have one, and which are worth the energy.
• What Are Max Battles and Dynamax?Power Spots, Max Monday, and what is Dynamax this season.
• What Are Type Matchups in Pokémon GO?The five multipliers, why dual types take 2.56×, and how to pick a counter.
• Which Mega Finale Path Should I Pick?Chespin, Fennekin or Froakie? The three Megas compared on damage.
• Which Team Should I Pick in Pokémon GO?Valor, Mystic or Instinct, and why the game's data says it does not matter.
• When Should I Unlock a Second Charged Move?18 families where a second charged move costs less before you evolve.
• What Should I Use the Master Ball On?It never misses, so what deserves it? And why catch odds aren't public.
• What Is a Photobomb in Pokémon GO?The surprise encounter, why Smeargle only comes this way, and what to photograph.
• What Is a Shadow Pokémon?The attack bonus, the defense penalty, Frustration, and when to purify.
• How Does Trading Work in Pokémon GO?Friendship levels, the 96% Stardust discount, and the 10 Pokémon that evolve free after one.
• When Should I Purify a Shadow Pokémon?101 Shadows worth keeping, 96 more worth keeping for what they become.

Looking for something specific? The Pokédex has a page per species, the tier list ranks every attacker, and the calendar covers what is running right now.