We often create tickets as agents for another agents in different team/department, so I implemented described behavior. I can share my code for anyone interested. I had to add just 5 lines of code.

yperano this "tickets belong to Users" is obviously not what reality shows.

Sure it is. The Agent is logged in as an Agent. They have access to the tickets in Departments and Teams that they have access to.

If they want to see tickets that they created for themselves they would have to log in as a User, not an Agent.

The whole problem is that osTicket is organised by Departments. This is not flexible at all.
One problem may be complex - it may include multiple departments.
What if a company has 1 person working 2 departments ?

We use Spiceworks for 8years.
osTicket is by far the better product with ONE Large But !

We have used spiceworks with very flexible organisation setup:
“If you are in a ticket, you can see it”
Agent can be creator, cc or assignee.
if you are one of the following you can see the ticket.
that way everybody can see any ticket if they are IN it .

I’m disappointed that I lost couple of weeks setting up osTicket

You got totally mixed up the logic. This is where collaborators or cc agents come in.
your organisation logic is wrong and not working for us at all.

I mean I am telling you how osTicket works.

Organizations are groups of Users, that can see each others tickets if you setup Autocollaboration.

Please please help send us the code

There is no coding required for what I have mentioned.

Your choices are:
Use Teams.
Or tell your Agents to log into their User account to see their tickets.
Or when transferring a ticket to another Department ticking the "Maintain referral access" check box.

Otherwise you are going to have to modify the core files to achieve your goals.