Glad to see you are getting started with Ardino-framework monitoring. The LearnEnviroDIY link walks you through tutorials starting with the most basic ones offered by Adafruit and other sources online. Then it targets what users will need for monitoring, where the crux is often learning how to manage libraries and how to use the Modular Sensors code wrapper for coding many operations that are cooperative and efficient for monitoring. The tutorial is designed to walk you through from 0 to 100, and depending on your prior experience with coding, code editors, and managing Arduino libraries, some users may not need to do the tutorial in sequence.
Your question about 3.3 vs 5 v power is highlighted on the first image in the first lesson of the LearnEnviroDIY tutorial. We did not detail the “how to increase voltage” because it’s a hardware toggle and it depends on the sensor. The sensors we use in the tutorial are all 3.3v, and users will consult the Modular Sensors wiki to determine their sensor/wiring needs for testing/deploying sensors. In short, there are several physical toggles on the Mayfly to switch the power assigned to each port. If you have a Mayfly in your hand you can see the labels on each of these toggles. Where to connect your sensor depends on communication protocol for the sensor, and LearnEnviroDIY touches on this in a few places, starting at Episode 6. We discuss OneWire and I2C in the tutorial Episode 6.
If you want to know more about the sensors already available in Modular Sensors and their wire/pin associations, check out he wiki: https://envirodiy.github.io/ModularSensors/index.html#mainpage_supported_sensors
The details of the hardware of the Mayfly are outlined here: https://www.envirodiy.org/mayfly/hardware/features/. Learning what the pin labels mean, if I understand the question, is something that people pick up when they do the Adafruit and other tutorials, though I haven’t had a student ask yet. In general D1, D2, etc are digital pins. Tx is transmit. Rx is receive. A is analog. GND is ground. 3V3 is 3.3 volts. Sw is switched power (I think).
If these resources are not “in a language you understand” I recommend staring at the beginning of the EnviroDIY tutorial, and especially thoroughly consume the resources that we link from Adafruit, because we intentionally did not attempt to re-create the tutorial content that they have.
Similar to doing your own home repairs, doing DIY monitoring isn’t a quick and easy thing. I personally spent several months getting up to speed before I recognized the need to create the tutorial, in hopes of helping others who are non-codes learn the ways of DIY monitoring.