1. You want a phone that has the 3g, 4g, 5g band coverage to be able to connect to carriers in Japan.
At minimum, band 1 3g and 4g 2100mhz.
http://anisimoff.org/eng/lte_bands/japan.html This will connect to ntt and softbank, two of the big phone companies.
Additional bands merely provide better coverage depending on location and carrier.
5g is so specialized, don't expect to use/connect on that unless you have 2021's latest and compatible phones, in general.
2. If you don't get a sim card directly from ntt, softbank, au, then likely a mnvo (virtual) company that operates off leased service from these three.
You'll need to know which of the 3 to check band coverage.
3. Some Samsung phones are known to be region locked. You have to insert and use a sim for the region it was sold in for an hour or so before it can be used with a sim from any other region.
Failure to run a local sim in a new region locked phone means it won't work outside that region with foreign sim cards.
IPhones - unlocked is unlocked.
4. Data coverage is almost always the whole country if your plan has it. No silly things like single city roaming only, in general.
Tokyo train stations have free WiFi, as do many other locations in the city.
https://www.wifi-tokyo.jp/ You could install chat apps (line, messenger, etc), even twitter, and get by that way for the basics.
If in America, you can get a free Google voice account with USA phone number on a pc. Then install the Google voice app pin the phone. When abroad and on free wifi, anyone can call you at that number free. You can call out to any usa number over wifi free as well. Text, too.
The Google voice app can work over data as well, using that up with calls and texts.
Usa T-Mobile customers on most Magenta plans have international text and data roaming free.
5. You do want to Test the sim for service asap. If you can't get any, likely a settings or compatibility issue you need to settle right away.
