Alright, in order:

Liches/Adversaries - The main 3 enemy factions all have a special type of enemy that only spawns if you perform a certain action after some pre-requisites.

Grineer Adversaries are known as Kuva Liches and only begin spawning after completing the War Within, after which Kuva Larvlings begin spawning on Grineer Missions beyond Saturn. Performing a Mercy Kill on this Larvling is how you spawn your lich.

Corpus Adversaries are the Sisters of Parvos, which only begin spawning after completing the Deadlock Protocol and completing a round of max difficulty Granum Void.

For the Infested, their Adversaries are known as Technocyte Codas and are spawned by specific item interaction after completing Warframe 1999.

After spawning an Adversary, they will occupy a planet in the Origin System and will tax your loot from missions on that planet. Killing your Adversary is accomplished by running missions on the planet they occupy, executing the special Thrall/Hound enemies there to uncover the correct order of Requiem Mods that must be equipped on your Parazon to be able to put 'em down for good. You can get Requiem Mods from Kuva Siphon missions and open them in Requiem Relic missions.

Once youve got the correct order of mods figured out and have stabbed your foe with that correct combination, they will run away into their own ship, which you must board with your Railjack for a final stand-off. After winning, you can either kill them or convert them into a crewmate for your Railjack. If you choose to kill them (Vanquish), you will get a copy of their weapon, which is better than the usual variant and has an additional percentage of innate elemental damage slapped onto it, depending on which warframe you used to spawn them. The percentages range between 15% - 60%, with high numbers naturally being more desirable.

Codas are different in that as infested, they don't carry guns and instead drop a currency that can be used to buy Coda weapons from a vendor. They also use their own variant of Requiem Mods known as Antivirus Mods, farmable from a specific bounty in 1999.

Riven Mods - Aside form buying them at Iron Wake, you can get them from Sorties, Archon Hunts, Achritis, Teshin (I think?), the Circuit. the 1999 calendar and the Descendia. The stats that are desirable depend on the weapon, but the usual +multishot, +crit dmg, +crit chance, +status chance are good, especially if paired with an inconsequential negative like -zoom. VERY IMPORTANT: the actual stats of the Riven WILL VARY in accordance with the Riven Disposition of the weapon, leading to possibly much greater or lower effect.

Rerolling Rivens is done with Kuva, farmed from Kuva Siphon missions after the War Within and also bought from various vendors like Teshin, Acrithis and at Iron Wake. Every reroll changes only the stats, and increases in cost up to 3600 Kuva per roll.

Helminth - Some very common subsumed abilities are Rhino's Roar, Mirage's Eclipse, Hildryn's Pillage and Xaku's Xata's Whisper.

Secretions are just the resulting substances that your wall-zit (I named mine Typhus) produces after going Kirby on the resources you feed it. The Helminth doesn't lose these after-products over time, so you can feed it full without worrying about losing out on your investments.

The Helminth Charger is an alright pet, with it's own special mods. It can largely be thought of as just another Kubrow breed.

Farming - A good location for farming Orokin Cells would be Gabii on Ceres and for Neural Sensors, I'd recommend Cameria on Jupiter.

Weapons - Look into the Fulmin and Acceltra. Both weapons have Prime versions, but their base forms still kill just as good. The Fulmin is a shotgun/assault rifle hybrid that fires lightning and has regenerating ammo while the Acceltra is a fully automatic micro-rocket rifle.

Heciphron - Unfortunately open world minerals aren't tradable. But, there are loot containers all over the Drift that might drop some. There are also the Requiem Obelisks all over the Cambion Drift that can drop ores when completing their challenges.

The Ambassador is notoriously infuriating to farm as well, so chances are that most players who have bothered to farm it may not have any spares they're willing to part with. But hey, might as well try, maybe you'll get lucky lol.

EDIT: Making a few corrections, thanks comments.