Angaran using Alliance tech??Silly cutscenes...

Are you trying to say the Avenger is better than any of the Kett weaponry they have been stealing for the last 80 odd years??  How many guns do you think the Exiles actually stole, and how many would they then let go of...you know a limited resource that they can't replace that may protect their lives??  Because they haven't been dealing with the Alliance, they have had limited dealings with the EXILES, who did steal some weapons, but how many??  And if they did trade some of them, why would the Resistance feel they were plentiful enough and good enough to use??

As far as ship design....are they really going to change the inside of their shuttles to match Alliance ones in a year, after building them a different way for unknown number of years??

Got to say, just play sloppy or laziness.

When I was studying engineering, one of Professors told us quite a thing: if any of the solutions was perfect, we would be using it and it alone. As it is, we have a number of solutions, because all of them have pros and cons.

It is especially so with arms and vehicles. We have sport cars, leisure cars, family cars, sub-urban cars, transport cars, armored cars, cars than can travel over water... We have hundreds models of pistols, sub-machine pistols, rifles, shotguns, machine guns...

Why are there Americans using European pistols, and Africans using Russian rifles? Because pros of some of those solutions are more important for them than their cons. Perhaps those arms are cheaper, perhaps they are more accurate, perhaps they are lighter, more sturdy, or simply more versatile.

My Ryder is usually using Ushior which is Angaran, or Scorpion which is Salarian. Jaal is using Lanat which is Kett. I see absolutely no reason why some of Angaran soldiers should not use Milky Way weapons or vehicles - if they find them better suited for tasks that they need them for.

As for availability... You can easily observe Yourself that Exiles have access to heaps of arms and vehicles. They had taken quite a few with them when leaving Nexus; they have stolen a lot from Nexus afterwards... And they are absolutely capable to produce even more. Please note that as per in-game lore majority of weapons are easily manufactured, which is exactly why we have Research and Development crafting system implemented into the game the way it is implemented.

There are some potential plotholes in MEA, sure; but this is not one of them.