Jackery Explorer 300 Plus solar charging goes through USB-C
The 300 Plus has no DC8020 solar port. Solar goes in through USB-C with an adaptor, and it tops out at 100 W.
If you are looking for the round DC8020 solar port on the Jackery Explorer 300 Plus, there is none. Jackery's manual says the 300 Plus is the first of its stations to use USB-C input as the solar charging input.
How to connect a panel
The manual shows a DC7909 to USB-C adaptor in the box. Jackery SolarSaga panels end in DC8020, so they also need a DC8020 to DC7909 adapter in between. The chain is: panel, DC8020 to DC7909, DC7909 to USB-C, station.
How fast it charges
The USB-C input is rated 12 to 27 V, 5 A, 100 W max. A bigger panel still works, it just charges at no more than 100 W. With a SolarSaga 200, the checker says works at up to 90 W in full sun.
Charge from the wall and the sun at once
The manual says the 300 Plus can charge from solar and the wall at the same time, gives priority to solar, and uses both up to the maximum allowed power.