I have a chunk of cash, so I'm learning how to best use it for generating cash flow.

I always tell new investors get at least a duplex because a SFH just doesn't generate enough income.

They are clearly being hired by someone to essentially cold call/lead generate. 

It's definitely worth a listen:https://www.passiverealestateinvesting.com/the-pros-and-cons-buying-all-cash/Basically you should do what you're comfortable with but you can generate more revenue, faster, if you finance since you can then buy more properties faster.

On the other hand, I was thinking about possibly doing one or two short term flips and using the profit generated from that to invest in my next multi-family.

The profit in apartments is generated by increasing the NOI not necessarily on the purchase.

I'm guessing we give each of them the numbers, have them generate us a rate and then just choose the best lender for that deal?

does not mean this will also happen to you.So this is what you do.YOU need to do a cost/deal calculation for every method for lead generation you are considering for your market.Now you know how much money you need IF you were to do DMM, or IF you were to do bandit signs.Now this is DATA, so you are not gambling much here.Then factor in the effort it takes and the time it takes for a deal.Bandit signs may be almost free but it is illegal and a LOT of effort and work, and again risk for hefty fines.DMM simply is a bad idea... so figure out what is feasible and then you go for that!

To be able to create from start to finish is an absolute dream but I have no idea where to start as far as being able to generate income throughout the entire process.

Money well spent to protect yourself and the hard earned investment income you are trying to generate, if any initially.