system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: I understand how much you are supposed to tip in the USA, but how exactly do the transactions happen?

Examples: 
Restaurant, bill is $25, so you want to tip say $5 and you hand over $40....do you say "just give me $10 back thanks"?

Same price for a taxi ride?

Also, say someone takes your bags up to your room, or similar service. Is it awkward to just give them money? As an Australian, this seems patronizing, and I want to avoid that.
gpt
At a restaurant you generally leave the money with the bill and then you can walk out, so you want exact change as often as possible. If you don't have it you can specify 'just give a $10 back' or whatever, or can get all your change then leave your tip on the table and clear out. For cabs it's generally just a case of asking for what change you want back. Bellhops? Yeah, just hand them something. 

If you're paying by credit most places will have room on the bill for a tip amount to be entered before charging or the machine will ask for a tip to be applied.