gistfile1.txt

To register:

➜  fortepan git:(master) ✗ curl -H "Content-type: application/json" -X POST 'http://localhost/fortepan/docroot/user/register?_format=json' --data '{"name":{"value":"test name"},"mail":{"value":"chx1975@gmail.com"},"pass":{"value":"test pass"}}'
{"uid":[{"value":14048}],"uuid":[{"value":"6494d669-086a-451f-b7e5-573ffb4e1dfe"}],"langcode":[{"value":"hu"}],"name":[{"value":"test name"}],"created":[{"value":"2020-02-14T05:18:53+00:00","format":"Y-m-d\\TH:i:sP"}],"changed":[{"value":"2020-02-14T05:18:53+00:00","format":"Y-m-d\\TH:i:sP"}],"default_langcode":[{"value":true}],"synonyms":[]}%

To log in (unpatched core requires a user name here but I patched it so users can use their email as well):

➜  fortepan git:(master) ✗ curl  -c cookie.txt --request POST 'http://localhost/fortepan/docroot/user/login?_format=json' --data '{"name":"chx1975@gmail.com","pass":"test pass"}'
{"current_user":{"uid":"14048","name":"test name"},"csrf_token":"AH8CpdwKyNdoTI89F3tUypGm470Qi9eQYYbeS-wK7is","logout_token":"NAH-F4C-yFPWJrex3cd14FpgMwv5TcH8tWFXLXDlTew"}%

To query a user, using "uid" from the previous reply, note the URL starts with "jsonapi/" and the Content-Type is "vnd.api+json" and the lack of "_format=json":
➜  fortepan git:(master) ✗ curl -b cookie.txt -H "Content-Type: application/vnd.api+json" 'http://localhost/fortepan/docroot/jsonapi/user/user?filter[uid]=14048'
{"jsonapi":{"version":"1.0","meta":{"links":{"self":{"href":"http:\/\/jsonapi.org\/format\/1.0\/"}}}},"data":[{"type":"user--user","id":"6494d669-086a-451f-b7e5-573ffb4e1dfe","attributes":{"drupal_internal__uid":14048,"langcode":"hu","preferred_langcode":"hu","preferred_admin_langcode":null,"name":"test name","mail":"chx1975@gmail.com","timezone":"America\/Los_Angeles","created":"2020-02-14T05:18:53+00:00","changed":"2020-02-14T05:18:53+00:00","default_langcode":true,"synonyms":null},"links":{"self":{"href":"http:\/\/localhost\/fortepan\/docroot\/hu\/jsonapi\/user\/user\/6494d669-086a-451f-b7e5-573ffb4e1dfe"}}}],"links":{"self":{"href":"http:\/\/localhost\/fortepan\/docroot\/jsonapi\/user\/user?filter%5Buid%5D=14048"}}}

To delete a user, we will use the self link from the previous answer, and the CSRF token from the login answer, that's necessary for every operation changing anything:
➜  fortepan git:(master) ✗ curl -b cookie.txt -X DELETE -H 'X-CSRF-Token: AH8CpdwKyNdoTI89F3tUypGm470Qi9eQYYbeS-wK7is' -H "Content-type: application/vnd.api+json" http://localhost/fortepan/docroot/jsonapi/user/user/6494d669-086a-451f-b7e5-573ffb4e1dfe

Now if we try to log in:
➜  fortepan git:(master) ✗ curl  -c cookie.txt --request POST 'http://localhost/fortepan/docroot/user/login?_format=json' --data '{"name":"chx1975@gmail.com","pass":"test pass"}'
{"message":"Sorry, unrecognized username or password."}

We re-register:
➜  fortepan git:(master) ✗ curl -H "Content-type: application/json" -X POST 'http://localhost/fortepan/docroot/user/register?_format=json' --data '{"name":{"value":"test name"},"mail":{"value":"chx1975@gmail.com"},"pass":{"value":"test pass"}}'
{"uid":[{"value":14049}],"uuid":[{"value":"ec86701d-1249-40b4-bd47-41b5c40b674f"}],"langcode":[{"value":"hu"}],"name":[{"value":"test name"}],"created":[{"value":"2020-02-14T06:46:38+00:00","format":"Y-m-d\\TH:i:sP"}],"changed":[{"value":"2020-02-14T06:46:38+00:00","format":"Y-m-d\\TH:i:sP"}],"default_langcode":[{"value":true}],"synonyms":[]}

Note the uid changed.

We re-login:
➜  fortepan git:(master) ✗ curl  -c cookie.txt --request POST 'http://localhost/fortepan/docroot/user/login?_format=json' --data '{"name":"chx1975@gmail.com","pass":"test pass"}'
{"current_user":{"uid":"14049","name":"test name"},"csrf_token":"dDjOtnn2KfcwOGjpSePQUKjDiEx-MYMkc47-XoYqsQ8","logout_token":"georBFlW6ypYBDcwGKjCpjI1iH3gneiwEfam6ytp3vA"}