To some degree, yea. I can make a conscious decision to focus on a specific task for 20 minutes and my mind will mostly have strings of thought that are associated with that task. But distractions exist; sometimes while working on a task I'll accidentally look up and see the 30 tabs I have open and start scrolling through them which initiates some other string of thoughts associated with those tabs. Transient thoughts like "what if vegetable was pronounced like regrettable" can happen and then some other strings of thought happen that are associated with that...

But I'm a fairly neurotypical person -- imagine for someone with ADHD or someone with OCD: they're going to have less control over their thoughts. I think how much control you have is probably a spectrum tbh