Oracle uses a lot of memory
But after 2-3 days of continuous work, the consumed memory increases ... 


Databases don't generally consume memory by themselves.  Even Oracle.  :-)

They do so on behalf of client Applications. 


  ... the number of sessions increases significantly to 450+. 


Sounds like those client Applications are opening connections and not closing them.

Have those Application(s) changed recently? 


  As far as I know, the database session should be thrown out by itself


[By default] The database has no knowledge of which sessions are eligible to be killed off and so will leave them connected for as long as they "want" to be.  Whilst it is possible to configure Oracle to kill off idle sessions after a period of time, this would only be a "Band-Aid"; you have a underlying [Application] problem that needs to be resolved. 


  I do not know how the database is configured. I did not install.  


Then ask yourself (very honestly): are you the Right Person to be dealing with this problem?  Do you have enough information to even look at it? 

I get the impression (apologies if I'm completely wrong) that you are an "Accidental DBA".  If that's the case, then I would suggest you need help from a real one, preferably the one that installed this database in the first place.  

If you can, go back and ask them for assistance.  

If you can't, then escalate this issue to your Management.  Switching your database "off and on again" every other day is not a viable, long-term solution to your problem - and it's not just your problem, either; it's a problem for your entire organisation and not one that you can "solve" in isolation.  You need input from the developers of those client Applications.