So in regards to building a list and actually placing asteroids, debris, and clouds what's the strategy? It feels like a crucial yet really lacking in coverage topic. And it feels like my biggest struggle. Like I know to keep rocks and debris away from droids if possible but what is the setup against aces? Swarms? Etc. And what pieces should my 3x1a resistance list work best with?

There's several strategies, depending on what you're playing, and what you're playing against.

Some lists want to create corridors/lines to force confrontation in, especially jousters and the like.

Some want a tight area to fight in, like making a "wheel" of obstacles in the middle.

Sometimes you want to make it hard for your opponent to turn in from one flank, which can be accomplished by placing a couple of obstacles as close to that side as you can, including one at 2-2, or 2-3(2 from the side between you, 3 from their side). Especially good for denying JM5K a turn in from their right side of the board.

If you're facing large ships, make the middle cramped. Likewise, aces want room to reposition, tightly placed obstacles can deny them their arc dodging.

Try to force swarms to split up, so they can't get all guns on you simultaneously.

Oh gosh. So. Many. Variables.

Tbh, unless I have a squad with specific obstacle synergy, I don't think about it too much. So pretty straight forward for me, most of the time. It's just so match up dependent, outside of those certain squads.

I'm not even sure if I'm even any good at it when I do think about it

Few examples though.

I have a list with Assaj, Latts and Unkar, so I create slightly crowded central areas with debris for Latts to hug and make sure to leave a few straight/diagonal lanes for Assaj to charge through and 5k. They can both Yolo debris, so as long as I get one of my big ones in or around the central area and keep whatever rocks there are fairly spread, I'm good.

Currently flying a Reaper, 3 x1s and Striker, so I'm placing 1 obstacle at 3x3 in the corner, to give me space to adjust when I move out. The rest I try to space fairly regularly, so I have room for multiple options and turns, since tight spaces are not good for them.

Flying Emon/Boba, I was all in on the magic roundabout. Big one middle, others circled around it. Drive in, block their obvious entrances/exits with bombs, drive out, shoot from the butt. Very basic.

If I'm creating a crowded area on the mat, it will often be because I have nimbler, more fragile ships than my opponent and will be able to use it to limit their moves towards me and create angles where I can dive on them.

I suppose my bottom line is that I'm trying to give myself as many options on the dial as possible, for as long as possible, while simultaneously presenting limited, or obvious options for my opponent.

On the latter, playing against A LOT of 4X recently, block flying, I present them a clear lane of approach and then tempt them into it. If they take it, great, because I've planned my escape and enfold moves at turn 0.

I'm not sure it pays to be too specific, the opponent is placing 3 of them after all and may not fly where you expect.... But general spread and corner distance, for deployment options, is definitely worth deciding on beforehand.

Mostly, I'm just winging it tbh, some squads don't seem to care greatly where they are

The key to placing obstacles is knowing how maneuverable your ships are compared to the enemy. What do you want to accomplish with the obstacles? What are your strengths and what are your weaknesses? A squad of Aethersprites or Inquisitor Ties will want a totally different set up than a Lando in the Falcon. Do you want to draw the enemy into the debries field and block him with mines or bombs? Do you want to fly around and pew pew from the sides?

A few rules I follow, when placing obstacles:

1. Do I need specific placement to use my ships to their fullest? Tie Defender wants long corridors to fly in. Ten Numb wants to be able to perform a Barrel Roll > Red Lock. Inquisitors and Strikers don't mind fitting into tight spaces. Falcon and Lambda want some space to maneuver.
2. What does my enemy need to use their ships to the fullest? As above, but you want to counteract their strategy. For example Han Solo (Rebel) will want to spread the obstacles around, creating a circular place in the middle he can use to fly in.
3. Does either of the squads have Bombs, Mines or Seismic Charges? Leave yourself easy exit paths, taking into consideration your squadron's mobility and maneuvers.

Second topic to consider is, what type of obstacles should you take? Again it vastly depends on what you are flying and how much it would affect your gameplan.

1. Asteroids. The ultimate road blocker, as four out of eight results hit you and you cannot shoot when you land on it. If you have a mobile and agile ships, I would honestly say those are the best. Can you cause tractor? Again the best choice. Not even mentioning Modified Ties or the Separatist Drones.
2. Debries Field. The easiest obstacle, imho. Affects you only on a Crit and gives you Stress. Nothing else. Beloved by named B-Wings and ships, that can easily shed Stress with Blue Maneuvers.
3. Gas Clouds. Lots f people love them for the free Blank > Evade effect, but I feel like since the change, they are much more punishing to Ace lists. Five out of eight results can seriously limit their survivability.

Knowing that, pick what benefits you most. Do you have lots of health on your ships? Gas Clouds are cool. Do you need lots of Agility Dice to survive? Perhaps Debries Fields are better then?

You also need to think about your ships, not just your opponent, and where you want/expect the fight to take place. If I fly aces and take gas clouds, I dont want all the gas clouds in a single area. I usually want one near the middle, one near my end and one near my opponents. That way I have a shield within reach if I ever need to bug out. And with aces vs lists like swarms or jousters, you also want to think about where you want that 1st engagement to take place and probably have some type of cover there for your ace to get that extra green vs 3+ incoming shots.

There are just so many factors, that there is no general obstacle placement anymore. With mining ties and droid swarms, you cant just say "Drag the swarm through the rocks to break them up" anymore. The best approach to any swarm/jouster is to approach at multiple angles, let them choose a target, then hopefully you placed an obstacle to help you out, and chose an area where obstacles are not going to bottleneck you in with the jouster/swarms and you left lanes of escape.

With so many options in obstacles to take, and lists that can avoid/brush off the effects, obstacle placement has gone from "Use them against your opponents" to "use them to benefit my ships and try not to screw me over in the process".