Please DO NOT modify your original post, as answers to it are confusing.
You can read a 3.3V sensor output with a 5V Arduino. However, you cannot control or send information to the sensor without using a level converter.
In general, for an arbitrary 3.3v sensor powered from the 3.3v pin, is it alright to read the output directly without a logic level converter?
The proper answer should be: "depends on the sensor". Which for whatever reason you're seemingly unwilling to disclose.
It matters whether it's just a simple digital on/off output; I2C or SPI interface; Serial interface; analog output; or something more exotic.
Here is a 3.3v SPI device:https://www.aliexpress.com/item/32870471046.html
That's I2C and indeed doesn't seem to have level shifters on board, so you'd have to add them externally.
A 0-3.3V analog output can be read using analogRead() without any issues, it's just that you have only about 70% of the range so you lose resolution, and stability may be an issue due to the two independent supply voltages (3.3V and 5V).
Depending on the sensor (yes, there we go again) and how it's all wired and powered (yet another set of variables) it may be better to use a 2:1 voltage divider and use the internal 1.1V reference.
A 0/3.3V digital output can be read with a regular digitalRead() call.
So... still no generic answer.
If the 3.3v is coming from the Arduino itself from the 3.3v pin, might there still be an issue with the different voltages?If the 3.3v is coming from an eternal source, but its ground and the Arduino ground are connected, might there still be an issue with the different voltages?
The limited A/D range when you connect 3.3volt devices to a 5volt processor can be fixed by connecting the Aref pin to the 3.3volt pin.If you do, then you MUST set Aref to EXTERNAL in setup(), otherwise you will damage the A/D.Now 0-3.3volt produces 0-1023.You still can input 0-5volt, but anything over 3.3volt will return 1023.
The issue is not so much the voltage level, as it is the stability of this voltage. Every 3.3V source has a slightly different idea of how much 3.3V really is. Changes in the load on the voltage source always affect the actual voltage coming out. The same accounts for the 5V source. And that in turn can affect the readings, but whether it does and how much it does depends on the power supplies and the way the signal is produced.