5000+ people:
- Company pays for airfare on their card, though you can have them put it on yours if you want the hassle.
- All other expenses are employee fronted then reimbursed, receipts only required over $75. Our reimbursement is generally faster than the credit card cycle so it's typical to be reimbursed before your CC bill is due.
- Meals & Incidentals are GSA per diem rates.
- Most stuff is booked through concur, but if you provide a minimal justification you can book hotels on your own. I used this regularly when there were charges to change airfares but not hotel and I was regularly going to a place where hotels were few and would fill up fast. I'd book at the hint of a need to travel, knowing I could cancel.
- We have company cards, but the employee still pays and is reimbursed, so few people use them.
- It's rare for me to book biz travel more than a week out, and I've had periods where I never flew an itinerary as originally booked, so a 3-6 week out policy would be a big fail. I once had to leave a message for my admin that I was heading to the airport monday morning and that he should book me a flight (with some prefs listed) and let me know which one I'd be on. That project also caused me to stop checking work email on sundays, because no good could come of it.
- We have an in house TA and after hours contract TA who are spectacularly good at IROPS. I tell all coworkers who are new to travel to make sure they program the numbers in their phone.

I send all my stuff to an admin who has an advanced degree in concur and have never had a report rejected for anything more than a receipt not attaching properly or an airfare not getting linked, usually fixed in minutes.

We used to get cash up front for travel - your admin would give you an envelope with your paper ticket and ~$100 or so in cash. That ended a long time ago.