I love reading military history, but after the end of linear tactics in WWI, I always have trouble picturing large unit combat, at least outside of urban warfare.
If I'm in the front line at the Battle of Kursk, and I see Army Group South advancing towards my foxhole, what does that actually look like? How many tanks can I see? How many men are in my field of vision, and how far apart are they? After a field army has broken through, what does their advance look like? Are the infantry marching in columns/riding wagons on the road, and if so, what's the largest single group you'd see doing so?
but after the end of linear tactics in WWI
If by 'linear tactics' we mean rows of men firing in volleys, no European Armies fought or at least trained in this way since 1866 or 1871 at the very least.
If you're referring to static/low-tempo, positional/semi-positional warfare the likes of the Western Front from 1915 to 1918, then this kind of fighting was very much a part of the Second World War. It characterized Monty's operations at El Alamein and the Mareth Line, as well as Anglo-American operations in Sicily and Italy to an extent, with an emphasis on firepower and the assigning of obtainable objectives. It was very much characteristic of offensive operations in Normandy, up until at least Cobra and Totalise, and it also characterized the Anglo-American operations against the German Frontier in 1944 prior to the Bulge, in the 'squeezing' of the Bulge, and the offensives into the Rhineland and across the Rhine. Soviet offensive operations from Kursk onwards, and even before, comprised a 'break-in' stage in which Soviet firepower, infantry and tanks would maul frontline units and eject them from their defenses, before shifting to a 'breakthrough' stage, in which tank armies and combined arms 'Groups' could be sent through to exploit the route.
If I'm in the front line at the Battle of Kursk, and I see Army Group South advancing towards my foxhole, what does that actually look like? How many tanks can I see?
Trenches, lines and lines of trenches, minefields, artillery positions, tanks hull down, camouflaged anti-tank guns; the Soviets established a formidable defense in depth, backed by what was at the time the largest theatre reserve the Red Army had ever assembled, Koniev's Steppe Front. Army Groups South lead with it's armour, so armoured fighting vehicles of every kind, APCs, Tanks, assault guns, etc would be on the move, and any infantry accompanying them would almost certainly fight in loose order, utilizing fire and movement.
The WW1 answers are great, as usual, but I am wondering about the second part of the question - it is something that has bothered me greatly, just what does "20th century warfare" look like? I don't mean at the lowest level, I can imagine a couple of men shooting so another couple of men can advance towards the enemy. But just one step above that, like OP describes.
How "broad" would an attack be? Like, would a WW2 unit deploy over a large swath of terrain, regardless of whether it's a field or a forest, with 20 tanks (I dunno) 10 m apart, 200 infantrymen 50 m behind them and 5 m apart, and move towards the objective? Or would they stick to the roads mostly? Would they form a chain of other units in the same division/corps many miles wide and advance together, or would the attacks be localized? How does a breakthrough actually look like? Is it just obliterating the defenders in, say, a two mile stretch of the frontline, and loads of mechanized forces pouring through this tiny gap 15 km behind the frontline to wreak havoc there?
I realize all of the above varied greatly depending on every factor possible but I just have a hard time picturing the "big picture". With line infantry I find it's easier, the battles seemed to be much more localized and "compact" for a lack of a better word, with rather "solid" blocks of soldiers maneuvering around. Even bits of WW1 in the west are easier to imagine due to limited mobility and the trenches (and plentiful answers on AskHistorians). But when it comes to picturing a, I dunno, a regiment or a division or more "attacking along the XY axis to take the villages of ABC and hill Z" I am absolutely clueless.