Running following script, you can get information about GOES-16 image products. Complete Image Collection has 388,607 images; one for each date (there are multiples images per day). On the other hand, there are 1,361 distinct dates (list begins in 2000-01-01 and ends in 2021-04-12; actual day). You can also observe that this collection can be filtered by dates. Finally, image with 'system:index' equals to '2020211203115800000' (from your question) is filtered by Metadata properties and its associated date is also obtained (2020-07-29T20:31:15).
var collection = ee.ImageCollection("NOAA/GOES/16/MCMIPC");
print("size of all collection", collection.size());
var allDates = ee.List(collection.aggregate_array('system:time_start'));
print("size of all dates", allDates.size());
var allDatesSimple = allDates.map(function(date){
return ee.Date(date).format().slice(0, 10);
}).distinct();
print("size of all distinct dates", allDatesSimple.size());
print("list of all distinct dates", allDatesSimple);
var new_coll = collection
.filterDate('2017-07-10', '2017-07-11');
print("new collection filterd by date", new_coll);
var img = collection
.filterMetadata('system:index', 'equals', '2020211203115800000')
.first();
print("image with id 2020211203115800000", img);
print("date of image id 2020211203115800000", ee.Date(img
.get('system:time_start')).format());