I have the backpack, but how do I put things in it?

[Tungsten Whitmarsh Cadow]

I put the backpack on the plus sign, but I don't see where I have more space. I still carry the same amount of things.

Also, what do I need to research to get straw?

[DarthNemesis]

Backpack upgrade gives backpack slots, not hand slots.

Press Q, comma, period, or forward slash to interact with your backpack in various ways.

https://autonauts.gamepedia.com/Straw

[Tungsten Whitmarsh Cadow]

The backpack seems useless now. What is it's purpose and when do I get to use it?

[DarthNemesis]

Its purpose is to hold more items. Unlike hand slots, you can mix and match different object types in a backpack. The player starts out with a backpack slot, while bots get them starting with Mk 1.

[Leost]

A couple more *tricks* with backpacks:

1. Tools fill your hands even though they are not considered big objects. But each slot in your backpack can have a tool in it. So for example in your hands you can only have 1 crude spade, but in your backpack you can have as many as you have slots (so 1, 2 or 4 depending what size backpack you have). This applies to bots too.
2. Heavy stuff doesn't slow you down if it's in your backpack. There are lots of 3 and 4 weight objects that will make you walk slow if it's in your hands, but won't if they are in your backpack. Use this when you can to make your tasks faster. Unlike #1 I don't think this applies to bots (bots don't seem to slow down based on weight).
2.a. Currently if you have something in your backpack and you pick up something really big like an 9 weight metal beam or something you can rotate your backpack (using the controls Darth mentioned) and the very big item that shouldn't fit in your backpack will...disappear). In your hands will be the item you put their originally. Once you empty your hands you can remove the "hidden" item like normal from the backpack. Benefit is since it was in your pack you still got to move fast with the item stowed. This is I am sure a bug and will someday be fixed.
3. You can do lots of good stuff with the ability to rotate your backpack items. For instance a bot, like you, can only have one tool at a time and the scripting for bots is terrible in terms of knowing what is in your hands, but you can plan your scripts knowing what is in what backpack slot and rotate items in the correct direction to have a single bot do multiple tasks with different tools all in once script. It's often better to avoid this, but in some cases I've found really elegant ways to get a single bot to do a job that would require multiple bots to shout back and forth to coordinate that would have been quite a mess in comparison. Likely this won't be useful until mrk2 or mrk3 bots since you likely will need more scripting room to really use multiple tools on anything anyway.

[Wat]

With bots, stashing spare tools of the same kind in the backpack, then retrieving them when the tool in hand breaks, is a simple way to reduce the number of trips to tool crates and increase productivity.