If I'm watching a let's play, I personally am not expecting the player to know everything. Part of the interest to me is watching a person newly experience obstacles and making mistakes, especially when I'm already familiar with a game that they're playing.

As for what to say, I think it could be something like just explaining your thought process out loud. See a tech/research Item that interests you? You could talk about how you might be able to use it, even if you are making incorrect assumptions. (One of my favourite youtubers quite frequently makes guesses at lore of horror games he's playing. Sometimes he's right, sometimes he's wrong. Either way it's something I personally enjoy.)

You could then also verbalize how you're planning to reach that tech, which lets your viewers know what you plan to do, and how you plan to do it. It could be something like:

I like Tech A, which means I need tech B to get there. But Tech B requires these science packs I'm not making yet, so I need more [item] to start that.

I wouldn't worry too much if you don't end up doing everything perfectly or according to plan. Getting distracted with some other project is a very common thing in Factorio, and I would assume most viewers would be able to relate to it as well.

All that said, don't feel forced to do so on my recommendation. It is definitely extra work, and it is really strange to talk to the air and record your voice when you get started.