Mass Effect 3/Geth and Quarians

General Hackett informs you that having turians and krogans (and maybe salarians if you made a deal with them) is not enough and believes the quarians have the largest air fleet. Also after the attack, the asari and the salarian council (if he's been saved by Thane previously) lends their support to the cause. Go to the meeting with the quarians. Tali would be available if she survives the suicide mission. Otherwise, you will be going along with General Xen.

End the Reaper transmission
You start out walking in literally no gravity along the long broken up tunnel. Continue on the path and up the ladder. Along the way, electrical shock waves comes in slow regular intervals on certain places. Another key area is basically an elevating floor which is shot by geth rocket crew. So continue until you reach the center of the signal. Perhaps like Tali, if Legion survived the suicide mission, he will be there. If Legion dies in the suicide mission, the Geth VI takes over. They may look similar but the Geth VI will keep denying that he is Legion. Anyway, take care of the geth ambush before making an escape to the geth fighter jet. By the way, you can agree of punish Admiral Gerreth for attacking while you and your squad is still on board.

Hacking the geth server
This mission is pretty much an easy mission. There's no enemies, allies and traps to speak of. All you have is a gun with infinite power after a few seconds. The idea is to shoot the orange beams on the first half, unlocking the quarian/geth history. The second is slightly harder so take of two orange beams before charging it into the data until you reach the end. A special note is if you have Legion, the extra part where you two meet will have an added significance. But the geth VI won't have it. Continue on to the end of this unique mission.

Save Admiral Koris
The first part is taking care of the geths first. The second is the anti-aircraft gun control station. Clear the geth before choosing your squad to shut down this station. Humorlessly, if you have chosen James as your squad, he'll just stomp the controls with foot, leaving you shaking your head. Along the way, you'll meet Dorn'Hazt, who's dying. The third part is taking down the AA gun station. On each control station, get one squad to disconnect while you go on the turret gun while the other squad cover you. The last part whether you want to save Admiral Koris or not, you'll use the turret gun on the shuttle after the takedown on the station, to cover the admiral from the remaining geths.

Note: You can do either two missions above but if you want to improve your war assets, doing both is worth the effort. Hit two birds with one stone but that's up to you.

Reaper Base Synthetic Style
You'll start outside the station. Take down the geths down the hill before climbing up the building to the top. You'll later reach an area where you can unlock the giant door, covering the reaper. Take care of the remaining geths before going to the top. Use your targeting weapon on the Reaper by holding the fire button until you hear the beep. Next is you on the hovercraft which is like a jeep with a gun turret which is a throw away moment. The last part could be the most tense. Lucky the targeting system hold its time when you let go of the fire button. The first is a free shot but roll and aim the reaper at least two more times. The last one is a timely moment, you have to be quick to aim, hold until the cutscene kicks in, and yes, this reaper can talk.

Side with Geths, Quarians or both
Now the easiest way of knowing why you can't have both is simple: If Tali and Legion didn't survive the suicide mission, you'll have no choice in this. If only Tali is there and Legion is not, but agree with the Geth VI, the quarians get decimated and Tali commits suicide. But if Tali and Legion did survive the suicide mission, agree with Legion and stand down the fleets to get a whopping amount on the war assets.

On the Wii U flashback comic picture story, if you choose to get everyone's help on Cerberus, resolve the conflict and save the crew, only Tali would be there. But if you go over the same thing but sacrifice the crew, Legion and Tali presumably resolve their differences will help with the impact of this war.