Treat concentration as a skill you build, not a trait you either have or lack. Start with short focused blocks and gradually lengthen them, protect one task at a time, and cut the biggest attention leaks first, which for most people are notifications and an ever-present phone.
The unglamorous basics matter more than any app: enough sleep, some daily movement, staying hydrated, and taking real breaks so you do not run on fumes. A short daily meditation habit also measurably improves attention over a few weeks. A timer helps by giving your focus a container and an honest record of the hours you put in.