Sure. This is going to turn out rather long.

I’m going to use Rome here since it’s the “standard” case but other civs developed quite differently, especially China which depending on who you ask had Feudalism very early (or around the same time, some might call the Three Kingdoms era a feudal transition). In addition, Rome in North Africa and Rome in West Germany were two very different places. But if you look you’ll find similarities in most transitions.

Concentration under the Latifunda (massive plantations) was a continual process. The Patrician senatorial families were socially prevented from working, even in merchant careers, so continual purchase of land and stocking with slaves was a key activity, and rich Plebian merchants would generally attempt to get in on the system where possible. This system operated differently in different areas (as a plantation in Libya and Egypt, but with soldiers who came under tough times and became tenant farmers elsewhere, especially in the north) but the result was similar to our network of large, vertically integrated companies that ostensibly operate in a market system. It was the largest agricultural surplus the world saw before the 1500s

The integrated nature of these communities firstly translated into political power in the centralised system, and infighting in the great families and their attempt to exclude the plebs from economic activity even as the plebs gained control of trade directly contributed to the fall of the republic. Augustus and the early empire attempted to dilute this power by consolidating political power in Rome into the military and granting conquered land to soldiers, but this only drove the centre of economic power away from Rome, as eventually all those freehold plots would be bought up, and the Senate was less powerful than buying out the provincial governor.

Crisis of the Third Century ended with Diocletian worsening the problem with his military state, by crippling the cities with taxes and allowing the large estates essential autonomy as long as they churned out supplies to the army. Then came the ending of the Classical Warm Period and a severe reduction of surplus, along with migrations.

The great estates folded in on themselves, those that weren’t taken over by (mostly romanised) frontier tribes. The cities starved and taxes dried up, Because the army relied on the estates to feed itself they began to be either dissolve or become local field forces. Since the landowners owned the economic base of the army, they could just ignore the imperial core and began to operate independent of it.

For agricultural developments, crop cycling, plow innovations, and a lot of agricultural cross pollination (some literally) due to the various groups of the migration period increased yields without requiring global trade networks or complex military engineering. This was not enough to increase surplus to the former level, but was enough to keep people fed.

For communication, remember that while most of the Romanised tribes could speak some latin, they couldn’t read it as well. Diocletian hollowed out Roman trade networks by turning them into military convoys, meaning a great deal of communication went through the provincial palace first. Finally, war, migration, and the reduction of urban populations meant that the infrastructure needed for messages to be passed were reduced. People simply needed to focus on the conditions around them to stay fed.

One lesson to take from this is that while the economic tools and systems and even theory is in place well before a mode of production falls, it usually takes a major multi-level crisis like the Migration Period or the “little ice age/wars of religion” to kick things off properly. Good thing we don’t have any of those looming.