SQL puzzles

I was asked what "=" and "IN" really do. Let us say we have two tables,
Positions(EmployeeID, JobID, Season, Year). Those who got a job.
Applications(EmployeeID, Year, Season, JobID) The applications for jobs.

If I say SELECT Positions.EmployeeID FROM Positions, Applications WHERE Positions.JobID = Applications.JobID; what does it do?
Does it select only the ones where there is, for every JobID in the Positions table, an entry with the same JobID in Applications?
If Applications had only one entry and that entry had an JobID = 6, then every entry with the same ID in Positions show.
How is IN any different?

What if we nest them?

SELECT Positions.EID
FROM Positions, Applications
WHERE Positions.JobID NOT IN --Filter out those who have the same JobID as in the temporary table created below
(SELECT Applications.JobID FROM Applications WHERE Applications.EID=Positions.EID) --the IDs where we have a match
AND Positions.EID=Applications.EID --Remove all entries that also do not have the same EID
GROUP BY Applications.EID
HAVING 3<count(Applications.EID);

Will it change if I change IN into =?

SELECT Positions.EID
FROM Positions, Applications
WHERE Positions.JobID != --Filter out those who have the same JobID as in the temporary table created below
(SELECT Applications.JobID FROM Applications WHERE Applications.EID=Positions.EID) --the IDs where we have a match
AND Positions.EID=Applications.EID --Remove all entries that also do not have the same EID
GROUP BY Applications.EID
HAVING 3<count(Applications.EID);