as long as "fundamentals" is not exceeding nyquist limit, there will be no aliasing.
In my opinion, this statement is a bit false.
But of course.
When the system does not produce an alias frequency from fundamental (aka 1st harmonic) then fundamental is not aliasing.
When harmonics are downconverted by ADC these down conversion products are there. And these down converted "false" signals we call a bit carelessly aliases. This is true with all harmonics, including also 1st. 1st harmonic aka fundamental.
When you drive some waveform that contains harmonics that cross the fNyquist "wall" these are converted down and these all down conversions are alias frequencies from these harmonics (every harmonic is harmonic but also we can handle higher than 1st harmonic as "its own fundamental". But down conversion products they are in this case aliases from these harmonics.
Take 11MHz "square" and look harmonics in system. 1st harmonic (aka fundamental) is 11MHz, 3rd harmonic is 33MHz and 5th is 55MHz, etc. (and now important thing, all harmonics, including also 1st (aka fundamental) can alias.
Now sample it using 50MSa/s.
There is fundamental aka 1st harmonic. 11MHz in this case not aliasing
There is now 17MHz which is alias (conversion) from 3rd
There is also 5MHz which is alias from 5th.
Fundamental is not aliasing. Yes this is now of course true. Still system produces aliases from these harmonics. In this case all other harmonics are aliasing but not 1st harmonic. (but then totally other case is how we see these aliases and there is big difference if we look in time domain or or in frequency domain.)
Naturally if also fundamental is aliasing then we see directly this aliasing and dramatic figures on time domain display. When harmonics are aliasing we see different square wave what is input. If we filter out harmonics what are higher than fNyquist also then we see different "square" what is our input but also different if we compare it to image what we get if other harmonics than fundamental is generating.
Fundamental (1st harmonic) aliasing looks very very different than higher harmonics aliasing. Specially in oscilloscope time domain (digital oscilloscope of course because analog oscilloscope do not have any ADC based aliasing.
Then... very different thing is what Sinc "reconstruction" do. It must not mix here with all fun Gibb's etc what must not mess with aliasing.
Then we come to the oscilloscope time domain display.
When fundamental is aliasing we can see its alias frequency. Example if fundamental is 15MHz and our fNyquist is 10MHz (sampling 20MSa/s) we can see 5MHz alias.
I think we all know that ADC can be used for frequency conversion. (as it is also used somewhere) And now where is way to think it works differently for "fundamental" and harmonics. Because there is nothing but harmonics. Every harmonic can thing as it is "fundamental" and it behaves just as fundamental. (just filter fundamental out and use this harmonic as fundamental.)
We can use 1GSa/s ADC so that we convert 995MHz signal to 5MHz but yes USB and LSB swapped.
If we are doing even step decimation it also works same way.
This is not for Siglent, not for Rigol, not for... long list of names. Brand names do not change physics or math, even if they want or hope. (except if they do mistakes or other way handle things wrong way)