Upgrading moodle 3.4 to 3.9

I am new to moodle and I need to do an upgrade. I have several questions first: 1) I have to upgrade from 3.4 to 3.5 and then to 3.9. Would I have to install all custom/aditional modules to 3.5 or can I do it just on 3.9. 2) This might be a dumb question, but I want to make sure I don't give anything for granted. In other to know which are custom/aditional modules I just have to go to the plugins view in administration and just copy the plugins that are on the section aditional plugins, right? Thanks in advance