Difference between unlocked sim-free phone and unlocked WITH sim?

I already know what unlocked is… I'm just confused about this other thing.
On Apple for example you can buy a sim free phone that is unlocked for full price
BUT
You can also get an unlocked phone WITH (for example) a T-mobile SIM for full price

So my question is, if I get the unlocked iPhone With the T-mobile SIM, can I still use any other GSM SIM card when I travel to other parts of the world to get signal and all? If yes can't I also do that with the sim free phone? I'm confused as to what the difference is… I'm using a GSM network in the US (T-mobile) and also a GSM when I travel to Europe for work (A1 Telekom)… Will I still be able to freely switch SIMs as I do now when I get to Europe and all with the unlocked Tmobile sim iPhone?

I think the T-Mobile one is somewhat limited in terms of GSM bands available to it vs an unlocked one with no SIM included.

I purchased the T-Mobile version for the iPhone 6 & 7 in the last 4 years & never used the included SIM, instead using my local carrier's SIM & never had any issues with calls/texts etc.

Had the unlocked, no SIM (SIM free) model been available at the time, that was the one I would have purchased specifically. Apple for some reason, in the US, don't offer that model initially, but do in other areas like Canada, the UK, etc.

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