Can the TX, RX, SDA or SCL pins on the Arduino Mega be used like any other digital pin? (I have seen in other threads that the TX and RX can be used but code cannot be uploaded. However I am uploading code via the USB cable. Does this make a difference?)

Sorry I searched and found the answer to my question. TX and RX can be used if no 'Serial.....()' commands are used in the code. SDA and SCL can be used normally but they are meant to be used for I2C components (whatever they are).

There are 4 serial ports on a Mega, the first one is used by the USB connection, so while you can use its pins for other things, such as for ordinary input and output pins, doing so will cause problems with uploading code unless you really know what you are doing.

The other 3 are spare for you to use as you please, so use as serial or as digital pins as required.

The I2C pins can also be used as digital pins as long as you don't also want to use them as I2C.

If you have to use those pins on a Mega, then you're probably doing something wrong.
If you need that many pins, then other solutions, like shift registers, might be better.
Tell us why you need to use them.

Can the TX, RX, SDA or SCL pins on the Arduino Mega be used like any other digital pin?

You could, but given how many pins the Mega has, why would you want to ?

I2C components (whatever they are).

not unlike serial, but much faster
components have addresses. they can be changed, within a limited range
in theory you can address up to 128 items. I had much grief when I got up to 7 items.

RTCs, LCD displays, OLEDs and weather sensors typically use I2C

Note: The SDA and SCL pins near the USB connector are connected to Pins 20 and 21. You can't use SDA and Pin 20 separately or SCL and Pin 21 separately.

The two I2C pins near the USB connector are for the convenience of shields that use I2C. They are connected to A4/A5 on the UNO and 20/21 on the MEGA.

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