And airgreg, don't speak unless you can at least improve upon the conversation
Broadsword,
Thanks for the advice. I would really appreciate any further help you can give me. </sarcasm>
If I don't take your questions seriously, it's because your questions aren't serious. You ask very random things without, it seems, any thought. I'll cut you some slack since you're probably 11 years old.
I'll take an swing at answering your original question, just to be nice. I'm no expert, but I would guess using foreign parts has something to do with one or all of the following factors:
1) Risk Diversification - if you had one manufacturer build an entire airplane, what happens if that manufacturer has quality control problems? Or gets bombed during a war?
2) Politics - foreign governments are known to actively lobby US politicians
3) International Trade - if we want Boeing to be able to make parts for other countries' aircraft, we might have to throw foreign manufacturers a bone every now and then.
4) Industry Specialization & Production Efficiencies - some countries are better at producing some things (i.e. Taiwan is very efficient at producing semiconductors, Japan builds good cars, the US builds good Walmarts...)
5) Cost - lowest bidder often wins.
6) Sales Incentives - it's easier to sell an aircraft to other countries if they get kickbacks on the production process. The Eurofighter is an example of an international production team:
http://www.eurofighter.com/Organisation/
I'm sure there are more knowledgeable people around to go into further detail on your questions.