Charging by power adapter

Yes. You can safely use an 18W USB-C Power Adapter, but you will need a USB-C to Lightning cable.

• USB-C to Lightning Cable (1m)

https://store.apple.com/uk/xc/product/MX0K2ZM/A

Your iPad will only charge at its maximum supported charging rate. As iPad7 does not support USB PD (Power Delivery) Fast Charging, the maximum charge rate is limited to 12W (2.4A @5V). As such, whilst the 18W Power Adapter can be used, you will derive no additional benefit.

as long as the volt are 5volts and the connector fits you can charge it by anything

Yes, but your iPad will NOT charge any faster than it is designed to charge, which, probably, around 12W, or a little more.

Base model iPads have no faster charging capabilities.