The process begins with me working with the players to select an interesting location and situation they’d like to adventure in. Usually, we narrow it down from two or three possibilities. Once that’s decided, I prep the campaign accordingly. Note, I am glossing over the prep part to get to the point where I can answer your questions.
When play begins, I describe the circumstances in which the players find themselves. They then roleplay their actions, and I adjudicate the results, which lead to me describing new circumstances. That cycle repeats itself throughout the campaign.
Let me be clear: NPCs (by which I mean both sentient beings and creatures) are essential to how I run these campaigns. The circumstances I present always include locations and the NPCs who inhabit them. If a player interacts with an NPC, I begin roleplaying that character, sometimes with voices and mannerisms, which I enjoy and perform well. the player responds by roleplaying in first person with me. It is up to them if they want to act, but it is not mandatory. Most players opt to roleplay a version of themselves with one or two quirks or mannerisms.
To manage this dynamic, since I’m just one person and the players are many, I often use a loose round-robin approach. It’s not a formal turn system like in combat, but it does reflect in-game time. For instance, if the party is camped for the night, I’ll go around the table and ask what each character is doing to for the evening, then another go around to handle what happens at night. I’ll typically spend 5–10 minutes roleplaying with each player before moving on. This helps keep the pacing manageable while allowing players to explore their characters and interact with the world. Players often pass if they have nothing particular to do. At times, smaller groups within the party will temporarily form, and I deal with that small group as part of the round-robin.