phone not switching on

This is usually caused by cheap chargers, such as dollar store charging bricks or 12V car chargers. They don’t offer any protection circuitry and can let high or dirty voltages through which damage the charge circuit on the iPhone logic board.

If you have already ruled out modular parts, then the only option is a logic board issue. This will require a micro-soldering repair. Look locally for a reputable shop that does micro-soldering repairs. Ideally, you want to find someone local but good workmanship trumps locality. There are many shops worldwide that do this so you shouldn't have any problems finding them. Talk to them and ask how they do their work. Focus on good practices like proper cleaning of boards once completed etc.

If you can't find one locally, then consider mail-in repair. There are several reputable micro-solderers patrolling this forum.