When designing hardware, you usually define what the expected operating environments are. Some typical environmental considerations are the min/max temperature, debris ingress, shock & vibration. If you know your product is going to operate in an area where material is likely to enter the product, then you can either try to keep that material out (sealing the product up), or make sure that dust entering the product won't cause failures (i.e. electrical shorts won't happen on a board by covering exposed areas with glue or making sure a mechanism can crush/clear particles). It's not necessarily more complexity in the product to navigate these constrains, but it is another thing to consider in the design.

For example, if you're making a phone that is going to be sold around the world, then you're going to worry about arctic/equator temps (will some of your components melt or ICs fail), salty sea air (will the product begin to corrode for people living by a beach), or fast moving elevators (will the speakers pop from a sudden change in pressure).

You can check out this manufacturers robot arms as some examples of existing products. They list some data sheets for their robot arms, including some arms that are IPxx rated. I don't think looking at robot arms is a 1to1 comparison for what you could expect from a humanoid robot since the considerations in the design process are going to be different.

website is kuka dot com/en-at/products/robotics-systems/industrial-robots/kr-agilus

Some are, some aren't.

For example, MIG welding robots tend to life a hard life. And if you look at photos of industrial painting robots, you'll find they're often fitted with plastic smocks.

If you look up photos online you'll only get marketing images from robot makers, where everything is shiny and brand new - I can assure you, it's not like that after they've been operating for a decade or two :)

Search for CNC videos, those machines work in oily, soapy, dusty and full of metal shavings environments and do fine.