How can I to know if my database MongoDB is 64 bits?

You can use buidInfo along with runCommand to check MongoDB for 32 bits or 64 bits. First switch your database to admin. Following is the syntax

use admin

After that use the following syntax to know if my server runs MongoDB 64 bits or not

db.runCommand(buildInfo)

Now execute the above syntax

> use admin
switched to db admin
> db.runCommand("buildInfo");

Following is the output displaying MongoDB is 64 bits

{
   "version" : "4.0.5",
   "gitVersion" : "3739429dd92b92d1b0ab120911a23d50bf03c412",
   "targetMinOS" : "Windows 7/Windows Server 2008 R2",
   "modules" : [ ],
   "allocator" : "tcmalloc",
   "javascriptEngine" : "mozjs",
   "sysInfo" : "deprecated",
   "versionArray" : [
      4,
      0,
      5,
      0
   ],
   "openssl" : {
      "running" : "Windows SChannel"
   },
   "buildEnvironment" : {
      "distmod" : "2008plus-ssl",
      "distarch" : "x86_64",
      "cc" : "cl: Microsoft (R) C/C++ Optimizing Compiler Version 19.00.24223 for x64",
      "ccflags" : "/nologo /EHsc /W3 /wd4355 /wd4800 /wd4267 /wd4244 /wd4290
      /wd4068 /wd4351 /wd4373 /we4013 /we4099 /we4930 /WX /errorReport:none /MD /O2 /Oy- /bigobj           /utf-8 /Zc:rvalueCast /Zc:strictStrings /volatile:iso /Gw /Gy /Zc:inline",
      "cxx" : "cl: Microsoft (R) C/C++ Optimizing Compiler Version 19.00.24223 for x64",
      "cxxflags" : "/TP",
      "linkflags" : "/nologo /DEBUG /INCREMENTAL:NO /LARGEADDRESSAWARE /OPT:REF",
      "target_arch" : "x86_64",
      "target_os" : "windows"
   },
   "bits" : 64,
   "debug" : false,
   "maxBsonObjectSize" : 16777216,
   "storageEngines" : [
      "devnull",
      "ephemeralForTest",
      "mmapv1",
      "wiredTiger"
   ],
   "ok" : 1
}