It is easy to keep muscles in maintenance mode.
Lets say you want to focus chest + triceps (which makes sense, many guys want to for whatever reason), I would do it like this.
Make up a 3 day gym program.
Day 1: This will be my heavy chest day. Start with Bench Press 90% of 1RM, which will take up 20-30min of my session. Then after that finish up with 1 more chest exercise, after that do 1 back, 1 shoulder, 2-4 arm exercises (2 for biceps, 2 for triceps do triceps after biceps to give them some rest time)
Day 2: Rest
Day 3: Light chest day. Start with bench press, do it at 60% of your 1RM , do it different, ie. dynamic mode, fast up, slow down, keep pauses top/bottom. Then do 1 light back, shoulder, 2 arm exercises. This session should have gone fast because it is easy and light. Then do 4 sets of squats, not too heavy, not too light, 80% of what you usually can do. Practice form and technique, that is all. If you want, you could have skipped the back exercise, and do deadlifts after squats, BUT dont go heavy on the deadlifts, the squats already worked your core and legs, so I would do deadlifts at 70%.
Day 4: Rest
Day 5: Medium bench press day, same with all the other exercises, do them at about 75% of 1RM.
Day 6: Rest
Day 7: Rest
See what I did? I focuses on chest by making it the 1st exercise every time, which is when you have your most energy. Doing bench press 3 times a week in that format sure as hell boost growth there if eating correct. But I also did the other exercises, and kept them in check mostly.