I've just got to the point where i can use robots. How do you go about ghost building stuff? Do you have to use blueprints? Can the constrution robots just build anything for you if the mats are in chests?

You should have gotten a tutorial presented when you got bots, that'll go over over how it works.

You can just place ghost items by hand (press shift) but not much point. The advantage is being able to smack down whole bunches of machines in blueprints.

They can build anything if the products are in provider chests. They have the exact items, they won't build from intermediate materials.

They build from the logistics chests, and yeah by blueprints. You can also have them tear things down as well, and they wind up in logistics chests. One thing I am doing is I have two separate logistics networks, and from my main I have conveyors taking things I want to the 2nd logistics network.

You can also start a Sandbox game mode to explore everything you can do (note infinity chests are extremely useful, you pick a item in the game, and it will produce as many of them as you want perpetually, then your inserters can pull those out into conveyors simulating different parts of what you'd like your factory to look like). Definitely recommend this approach, you can explore as much as you'd like to explore.

Also note, there are a few websites with prebuilt blueprints as well.

It's possible to order bots to put out things for you without blueprints. You need a personal roboport, and some construction bots in your inventory. It's been a while since I played, so I don't really remember if you just place with left click, or some modifier and left click. It vastly increases the range you can reach when building manually and its an excellent qol upgrade. The more construction bots you have (up to the researched limit) the more faster it'll be, because they have to return to you to pick up the next piece.

I forgot to mention in my comment, if you hold shift when putting something down, it will ghost it. So you can have 1 of every item you need, no need for a personal robot port, and they will ghost it. The great thing about letting bots build is if you don't have all the materials they will drop it as it comes into the logistics network!

Edit: fixed a typo

Yeah, ghost placement is what I was thinking of, thanks for the reminder! The approach you suggest requires being in the range of a logistic network however, while carried construction bots are only limited by the range you have available from the engineer.

There are tons of designs you can do with bots, some have built completely belt-less bases using them. They're limited by their speed and range however, so you always have to balance to the constraints of the tools you use.

You can copy existing sections of your factory with Ctrl-C and paste ghosts with Ctrl-V. There's also a paste buffer that you can scroll through with the mouse wheel.

You can also cut + paste

It takes a lot of the tedium away when you made a miscalculation and need to be just a little bit closer or further from something.

Now you can cut, move over 1 and paste.

I've also found it really nice for rails. If you want a t-junction, just go away from everything and make a 90-degree rail curve. Then cut it.

Then go back to where you wanted the t-junction and paste in the curve and rotate it and paste again to get perfectly lined up sections of track.

and then you can have blueprints and upgrade planners and destruction planners.

upgrade planners let you convert say all slow belts to fast belts and leave the inserters alone.

destruction planners let you remove only walls but leave the electrical towers and rails alone.

it just goes deeper and deeper.