Gluing is seldom needed/used these days. How the process is done exactly will depend on your fab.
But most will first populate the high-density side and solder it, and in a second run, the other side will be done. The components on the high-density side will usually be held by their surface tension. The manufacturer will also slightly change the temperature profile for the second side, so the higher temperature “capacity” of the high-density side will keep the components in place.
Some packages can not be reflowed twice, because of their high-temperature sensibility. These should be placed on the side soldered last.
But the most important thing is, talk to your manufacturer, they can help you and very often can provide you with their assembly/layout guidelines. If you follow them it will save you a lot of money and trouble in the end. Proper PCB assembly is not a totally straightforward process. It requires communication between the customer and the assembler and a lot of tweaking.
Read More: How Does BGA Soldering Work
#PCB Assembly
