Is this the link? International Plans | Traveling Abroad without Roaming Fees | T-Mobile
If not this one states;
The T-Mobile ONE, Simple Choice, New Classic and Select Choice plans provide unlimited text and data in 140+ countries and destinations. Log in to My T-Mobile to confirm that you have one of these plans. If you have one of these plans, no need to add a separate feature or service to your account or call Customer Care before you travel. International roaming with unlimited data and text is now included in qualifying plans at no extra charge.
So looks like you are good there as long as I am assuming you have a post-paid account as pre-paid don't get international coverage.
For what countries and coverage within the country just enter your country here and it has the details.
Now remember data is 2G generally at best, ok for navigation and slow facebook posts but you won't be watching any videos or anything. Now some countries or carriers don't limit you to 2G even if that is what they put in their roaming agreements with other carriers but for the most part it will be pokey, unfortunately those countries that don't bother with the limiter are few and far between.
One thing not advertised as much but if you need to call back to the US or have someone call you from the US that yes why on cellular it is $0.20 per minute but if you use Wi-Fi calling and are calling/receiving a call from a US number those are free. When I am out of the country the kids or family know to text us they need/want to talk and we will let them know when we have access to Wi-Fi (like at the hotel or restaurant) and can call them for free :-)
Also remember this phone number if you are having issues, this is TMO's international help number and is free from your cell phone +1-505-998-3793
As for unlocking your phones, this link should have all the answers you need. If you meet the requirements it explains what the procedure for requesting the unlock code is;
Does your phone have the T-Mobile Unlock app? If so, you can request the unlock via the app.