In the Nano Every schematic, PF2 and PF3 are connected over to PA2 and PA3 respectively. That is, Arduino pin A5 and ATMega4809 pins PF03 and PA03 are connected together by PCB traces such that there's no distinction between them, PF03 doesn't have its own header pin anywhere distinct from PA03. They're both A5. Likewise for Arduino pin A4 and ATMega4809 pins PF02 and PA02.
The Nano Every was designed to be compatible with the original Nano boards that used the ATMega328P. On these boards the A5,
analogRead-capable pin, is also the I2C/Wire clock signal. Likewise,
analogRead-capable pin, is also the I2C/Wire data signal.
On the ATMega4809 the I2C/Wire Master/Slave signals on PORTA routed to the A5 and A4 pins are not also
analogRead-capable. That is, they are not connected to the analog-to-digital converter in the chip as they were in the Atmega328P on the old Arduino Nano boards.
To mirror the workings of the original Nano, where A4 and A5 were both
analogRead pins and I2C/Wire Master/Slave pins, they connected A4 and A5 to pairs of ATMega4809 pins. One of these pins on PORTF is connected to make the Arduino pin
analogRead-capable and the other pins on PORTA are connected to make the Arduino pin Wire/I2C Master/Slave capable. In other words, they used two AVR pins to synthesize a single Arduino pin capable of both functions that were available on that pin on the older Nano boards.
When it comes to using PORTMUX to switch choice of I2C pins, practically speaking, you won't be able to if you're looking to get alternate pins for TWI Master. PORTMUX lets you switch the I2C Master/Slave or just Master peripheral onto pins PC2 and PC3 instead of PA2 and PA3. But, in the Nano Every schematic, you can see these are not connected to anything. So if you wanted to do that, it would mean something like soldering wiring-wrap wire directly to the MCU pin, which would be difficult. And PC2 and PC3 are the only two other ATMega4809 pins that PORTMUX will route the hardware I2C Master/Slave peripheral to. So, practically speaking, if you want I2C any pins other than A4 and A5 on a Nano Every, it means using a software (bit-banged) implementation of I2C.
You asked (in comments) about the TWI support indicated on PF2 and PF3 in the ATMega4809 datasheet. I think I'd either been looking at an older datasheet or maybe disregarded those pins because of how they're hardwired over to PORTA on the Nano Every. The ATMega4809 has a "Dual TWI" mode apparently, where you can have just master mode on the pins that are normally both, and just slave mode on an alternate set of pins. In two configurations of PORTMUX under Dual TWI, the PF2 and PF3 are where the slave-only capability of the peripheral are broken out. With PORTC's TWI pins unconnected and PORTA's TWI pins hardwired to PORTF's TWI pins, it's unclear to me what advantage you could get out changing PORTMUX on an Arduino Nano Every.