re: "My company is new to Upwork how are most freelancers trained on company systems and procedures prior to starting a project?"
Most freelancers come to Upwork with an already-established skill set. They are available to work for clients using those skills.
Most of the work done by freelancers on Upwork does not involve any training on company systems.
There really is no "standard" for how "freelancers are trained on company systems and procedures."
If you would like to hire freelancers to work on your behalf using company systems and procedures, that's great!
Many clients use Upwork for doing just that.
You have the flexibility to do this in whatever way works for you, as long as you follow Upwork ToS.
The most straightforward thing to do would be to post job descriptions describing the kind of work you want freelancers to do, and state that the work will require some training, which the freelancers will be paid for.
Then hire freelancers using an hourly contract to participate in training. If you want to use separate contracts for training versus post-training work, you may do so. But it is not necessary.