How did the Archon gain control of...

He just hacked him, shut him down and shackled him with his tech. Sam maybe a power full computer but his interface is limited to connections to people.

What point would it serve him to attack the nexus?  that's like stopping and doing a dance right before goal in football with out crossing the line.

I still simply think it's BW bullshit bc SAM can detect the archon's device or whatever in Ryder's bloodstream. Ryder obviously can't notice the device, but SAM does, so SAM's detection is not limited to Ryder's senses.

Also, why can't SAM shut off his connection to the Ryder sibling to prevent the Archon from using the sibling? Or he could kill the sibling (just saying good of the universe and all). How did the Archon gain such intimate knowledge of SAM? Did he take advantage of the other SAM on the salarian ark? Wouldn't we have known he tampered with it?

Also, how is the archon able to use the remnant tech so well without any practice? He can use it better than Ryder can despite his own ignorance--proven by his own inabilty to get it working and collecting the equivalent of remtech junk on his shop.

Honestly, I really hated the last part of the main quest bc BW doesn't do a good enough job making the archon's scheme all that plausible. That and the archon was boring as shit, evil for evil's sake, predictable snooze fest.

That doesn't really answer my questions. Yes, the archon uses sibling Ryder, but how for all the reasons asked in my previous post. And SAM is not limited to what Ryder feels; there's no way Ryder could know archon implanted a device inside, but SAM knew. Otherwise, how is SAM able to determine all the environmental changes in planets when we activate vaults? Ryder can't determine the temperature is now exactly 78.9 degrees.

"Just because" archon does/can isn't good enough from BW. Basically it comes down to the fact that the game fails to demonstrate the archon even has the capability to accomplish what he does in the game. It's like the reasons he can are "alternative facts."

The Archon "shackled" SAM. For all its personality, SAM is an AI; and shackled AIs have no choice but to obey directives.

Of course SAM has sensors. It has ridiculously good perception through Ryder's hardsuit and omnitool, and likely some camera access in SAM node. It has no defenses, though. Most people are freaked out by SAM. They reticently accept that the Pathfinders use SAM, but aren't particularly comfortable with it. There's no way they'd allow SAM access to hardware with which it could battle "intruders", fearing it would possibly turn them against AI personnel. Once Hyperion falls under kett control, SAM is relatively defenseless against the Archon's shackling.

We have no idea just how intelligent Archon is, but he seems likely much smarter than Ryder. He's old, possibly very old, and thus pretty experienced. He has the genetic traits of who-knows-how-many species. His having the smarts to puzzle out Remnant tech isn't that shocking, to me. He's been studying this stuff for 80+ years. He has a grasp of how it works, but can't access it at all until seizing SAM.

More importantly, we learn after the fact that he wasn't even controlling the tech during the fight. It just seemed that way to Ryder. He was completely overwhelmed by the connection to Meridian. His snooping awoke the damaged Architect and its bots, but he wasn't controlling them.

I can't help him on the "being more interesting" front, since that's entirely subjective.  The other issues, though, actually make sense once all provided data is considered.

1. On SAM it's been stated over and over how sam does his thing. Ryder is a relay and his brain is connect to sam due to the implant. SAM uses Ryder's senses and implaint to connect to the world . THIS WHAT SAM LITERALLY TELL WHEN YOU ASK YOU THIS INFO. this is not me making bs up. This is what SAM and the plot says. And we know that SAM can sense what's going in Ryder's body because he tell you and we see it on every planet with any hazard. He ch he even turn off Ryder's heart and tells Ryder's what his bodies condition is. If it have to I will literally post video with his stating this and codex stating this.
As I said before SAM is not Edi, he is limiter to Ryder, Ryder's senses and Ryder's implant for any sensory.While Edi's sensory is to the normandy and her robot body, SAM just has Ryder and ryder's implant. He can just shut down, that is done by a 3rd person and that would kill Ryder if he did it.
In short, sam is comment to Ryder due to the implant and Ryder is a relay for sam. THEY ARE CODEPENDENT.