Terminology thing: Roll20 calls screens/maps “Pages”. When we move to a new “page”, it basically means moving to a new area or scene in gaming terms.

We're going to try doing a combination of running with maps/tokens and theatre of the mind. This is close-ish to how Colville-senpai normally runs and I'm copying everything from him.

First to get out of the way, maps will exist for *most* areas (certainly not all). When a map exists, so does a grid (5ft cells).

Secondly, all battle encounters of any level of complexity will use tokens for both PCs and monsters, regardless of how detailed the encounter map is.

All right, so like… we'll try running with 3 different types of concepts for environments: Overworlds, Dungeons, and Encounters.

Encounters of potentially high complexity (many rounds or many enemies) will move to their own battle page (like a JRPG). This battle page will be custom tailored to the battle, but not necessarily highly detailed (maybe sometimes). Encounters may or may not use fog of war. Just depends on encounter and environment.

Overworld pages are anything that happen outside of a dungeon crawl scenario (like towns, roads, fields, or basic woods). These may or not be mapped out, just depends. These will not use fog of war as they aren’t dungeon crawl scenarios. I probably won’t put enemy tokens on the overworld maps unless very notable, but I will describe the environments and potential encounter/ambush locations if they exist. If I say, “there’s a large bush to the right of the road”, it’s your guys’ responsibility to mention that you might check the bush somehow. Otherwise, for a normal encounter with no elements of surprise, I’ll just describe (for example) that a gryphon is sitting in front of you and if its a situation of hostility, we move to an encounter page and roll initiative.

Dungeon crawls (caves, castles, dungeons, forests, etc) will use basic maps. Any unique or defining aspects of a dungeon room will likely be reflected on the dungeon map, but detailed aspects (”the chair in the corner”, “the pile of bones next to an urn”) will usually not be mapped. Dungeons will use fog of war, illuminating only the room that players are currently inside of.
Monsters will sometimes be reflected on the dungeon map, and sometimes not. Like overworlds, it depends on the encounter. Unless under extraordinary circumstances, you’ll never get unfairly ambushed out of nowhere - I want to keep this type of thing fair. Just remember you can check environmental details I describe even if they aren’t reflected on the map.
Also, if players want more detailed maps of dungeons, they are free to keep them themselves.

Ultimately, remember DM description > map detail. If I say there are 5 beds in a room, but the map shows 1 bed, that 1 bed is just symbolic, there are 5 of them. Maps are there as a tool to help us in describing stuff - they are not considered authoritative. Again I know I'll need to avoid gotcha situations with this.

Finally, just because we move to an encounter screen that may have less environmental detail reflected on the roll20 map, it doesnt mean the room itself changes. So… if I say “there’s a pile of bones next to an urn”, and we move to an encounter page while staying in that room, that pile of bones and urn are still there for that encounter, and you might use them if they’re somehow relevant. They just won’t be drawn out on the roll20 map for you (probably).