How to use pagination in complex query

Thank you so much,
Your explanation makes sense, and good one, now I understand what model used for, I assume model is active records keeps all data till the browser is closed, is this case if I have 100 users online means I have 100 open connection , is that right ?
Also, can someone show me to use QueryBuilder as above query as join tables, and use where
I know how to do it within one table as online doc explained, but I can't find out multi tables example with conditions
I'm learning it as I go with my project
Thank you again