I feel like no one reads... if you order through Apple do they lock your phone like the carriers do if you order a Carrier specific model(ie. a T-Mobile compatible model).
What Mr. Willie is saying is that whether your buy your phone from Apple directly or you buy it from your carrier, if when purchasing the phone you finance it using your carriers financing plans, the phone will be locked once activated (except Verizon I believe).
So for example, if you have AT&T as your carrier, and you say walk into an Apple store and buy an Iphone Xs Max for AT&T, and instead of paying full retail price for that AT&T phone, you decide you finance that phone using AT&T's NEXT financing, even though you are buying it at an Apple store, the phone will be locked to AT&T during the activation process, so yes, both sims will be locked to AT&T until you have paid it off.
If you decided to walk into an Apple store and purchase that same phone using Apple's financing plan like the Apple Upgrade Program financing, then your phone would NOT be locked to AT&T since you are financing it with Apple directly, and not your carrier.
If you walk into that same store and purchase that same phone at full retail price, then your phone is unlocked
Hope this helps you.....