I thought after kills you get loot. I have only got loot maybe twice in over 50 bot kills. Do need to be on top of it or something. Those cans fly right by me really fast.

Are you talking about hive bots? Because you need a mission to get money for that

If you mean the convoys, you have to blow up the trade ships to get cargo:
Behemoths, Atlases, Centaurs, Wraiths, Valent-Buses

Not: Centurtions, Warthogs, Vultures

If by "loot" you mean cargo that can be sold, those cans are it. You have to run them over to get them into your hold. You stop being able to pick them up when your hold is full. Hit j in flight to manage your cargo, and use the cargo tab when docked to sell or move cargo.

I'm talking about loot from combat training mission bots. I also blew up a transport ship and got nothing from it. The cans just seem to fly right by me real fast so i don't get them.

Most likely reason? your ship's hold is full already. Press 'o' or 'j' inflight to verify you have cargo in the ship.

you can target them by pressing x if they are in front of you. And it are th epurple dots in your radarscreen.

A full cargo hold will do that to you everytime. You can fly over the crate over and over but if there isn't room in the hold you won't pick it up.
j opens the jettison menu in flight if you want to drop less desirable cargo to make room for something better.

... and, cargo does not fly past you... you fly past it.

also, and this is just a minor point, but a full load makes you heavier when your fighting and if your doing combat mission, just collect the crates when your ready to head back to heal or take a break. There are better ways to make lots of money, that's all just icing on the cake really.