This is a general overview and was written several updates ago, to see the most updated information check the Change Notes tab.

All triage centers/first aid stations/medical bunkers now have a repair aura to cut down on building slots
Some faction engineers (UK Sappers) can still build their repair facilities (for now, not sure what to do with them)
Each faction can generate munitions from their medical truck (2.5ton truck for Axis, CMP truck for Allies) every 60 seconds. 4 possible upgrades, starting at 15muni/60 seconds going to 60muni/60seconds

Unit upgrades (i.e. weapon packages) have been expanded
Most if not all units can get every weapon upgrade they have (i.e. Tommies, Jaegers)
Rough overview per faction:
DAK:
DAK unlocks weapon and battlegroup upgrades by researching their armory upgrades, which unlock a different weapon type per tier.
Tier 1: Mobile Assault Munitions (MP43s and MP40s)
Tier 2: Light Infantry Munitions (G43s)
Tier 3: Heavy Firepower Munitions (Panzerfausts/AT Satchels)
Note that AT Satchels are not currently limited by this upgrade. This will be changed in future updates.
-Palmgrens are generalists able to get mp40s or g43s and an lmg
-Panzerpios can get scoped G43s
-Agrens get MP43s (plunder mp44 stats)
-Base Construction Panzerpios get G43 package (same as Palmgren) and PzB AT rifles
-Guastatori get a captured M9 Bazooka
-Panzerjaeger can get their Tank Buster Panzerschreck upgrade as well as a 2nd Panzerschreck at veterancy 2
DAK Notes:
-Agrens are 6man by default and scale harder
-DAK Munitions generating unit is the 2.5-ton Medical Truck
-DAK shares officers with WM (Sturm Offizer, Coastal Officer)
-DAK HMG bunker replaced entirely with WM bunker with full upgrades access

USA:
Each of USA's support centres has 1 battlegroup ability unlocking upgrade, and several related upgrades since USA has quite a few battlegroup passives.
-Rifles, Engies, all Scouts can get M1 Carbines to replace their Garands
-Rifles can get up to 3 BARs per squad (1x BAR package changed to 2x)
-Assault Engineers get Thompsons and a Heavy Assault Kit, making them much better in combat
-Paratroopers can get both 2x 1919s and 2x Bazookas
-Rangers can get 2x 1919s and 2x Bazookas (with Ranger Weapons Training support!) so they're not at the mercy of crate RNG
-Rangers can also get Garands if you're weird (does not take up slots or interfere with other upgrades, only replaces Thompsons)
-Resistance Fighters are made out of paper but have a lot of damage. Kind of like opposite day Coastals. They can get 2x Breda LMGs, M38 SMGs and a Bazooka.
USA Notes:
-Lieutenant added: 6man stronger Rifleman. Max 2. Can fire its LMG on the move and get a Bazooka. Heals in an aura out of combat. has Stuart commander aura at vet 1, captain experience aura at all times. Follows closer than other officers.
-Cavalry Riflemen added: M1 Carbine wielding Riflemen upgradable with Thompsons and a Bazooka. Like cheap Rangers or USA Agrens.
-Start with Engineer
-Can build 1 of either Pathfinders, Artillery Observers or Cavalry Scouts. Scouts are capped to 1 (shared cap!) until the relevant support centre is researched. It should be obvious but just in case: Pathfinders = Air/Arty Obs = Infantry/Cav Scouts = Mechanized
-Can build all 3 support centres (recommend swapping between them by clicking on the icon to the left of the HQ building icon repeatedly, it can be a little annoying)
-build UK emplacements with Aussies in WSC
-have access to UK tech tree through Aussies if you want but I'm not supporting it if anything goes wrong (if you want access to a specific unit you can ask)

UK:
UK battlegroup abilities are in the training center (where you buy the veterancy upgrades)
-Tommies can get all upgrades
-Tommy Bren upgrade split into Bren Officer Package (Max 2) and Bren Medical Package (Max 3)
-Note that both of these upgrades count as a kit for ability unlock purposes
-Caps are kind of arbitrary tbh
-Officer package turns squad into equivalent of USA Lieutenant: buffs infantry in radius and increases xp gain, with no heal aura
-Follows close like Lieutenant
-Medical package heals in an area around the squad like a med truck
-Base Construction Sappers can become Heavy Sappers with their kit, turns them into assault infantry
-Guards can get Lee-Enfields (if you're weird) (same rules as Ranger Garands)
UK Notes:
-4man 2 PIAT teams available from section command post
-SAS LRRP Squad available (6man Lee-Enfield commandos with a Vickers K, combined arms bonuses with vehicles)
-Company Command Section available (4man officer squad, supervises sector, buffs emplacements, constructs emplacements, similar to Coastal Officer)
-comes with Bren and Recon package
-UK has a secret tank unit only unlockable through research. It's kind of horrific though.


WM:
WM battlegroup abilities are in their HQ upgrade panel. Their medical upgrade has been granted to them by default. Over Repair is in the Infanterie Kompanie.
-Base Construction Fallschirmpios can get FG42 replacements for their rifles, an MG15 and a Panzerschreck
-Sturm Offizer comes with a Panzerschreck (no cost) but has to get his kit to sprint
-Coastal Reserves can replace their Carcano rifles with Kar98k's for better performance at close range and on the move. At higher veterancy levels can get Panzerschreck and Breda LMGs.
-Grenadiers can become Assault Infantry by getting their Assault Package, or get an MG34 LMG and then a Panzerschreck at vet 2


WM Notes:
-Sturm Offizer has -15% damage reduction at base, and my assumption is that his Light Assault Kit will add to that (making it -30%). I don't know for sure though and haven't tested
-Oberschutzen (Veteran Grenadier) available: Much stronger (100HP) Grenadier squad with better weapons (fspio kar98k), a better sprint and access to buffs. Can get an MG34 LMG and a Panzerschreck.
-Grenadiers have bunker free reinforce (coastal passive)
-Panzergrenadiers get a Panzerschreck
-Panzergrenadiers have the most infantry AT on a squad with a Panzerfaust, AT Satchel (kit required) and Panzerschreck if desired
-Stosstruppen get Sturmpioneer kits (name/icon does not change, unless someone knows a good sturmpioneer icon in the game files that isnt already being used)
-Cloak in cover, AT Satchels, Flamethrower ability etc (kit abilities)
-G43s replaced by MP44s, keep MG42 when upgrading