Does the RC300 (or any audio device in general) have a special internal gain for the XLR input? When using a dynamic mic via JACK the sound level is very minimal, whereas when using it with the XLR cable, the sound level is good. I know standard audio interfaces have a gain knob for the XLR/JACK inputs, but since the RC300 does not, I was just wondering how it works. Thanks!
Did you mean the Boss RC300 loop station which does have gain knobs for XLR and ¼” jack inputs (labelled MIC & INST respectively) or something else?
In general for audio devices where both XLR MIC and jack inputs are provided you can assume a MIC XLR is intended for low output low impedance balanced microphones and an INST (instrument) jack is intended for a high output high impedance unbalanced signal such as a guitar pickup.
The Boss RC300 manual quotes (page 43) nominal input levels of -50 dBμ (~2.5 mV) into 4 kΩ for the MIC XLR input and -10 dBμ (~250 mV) into 1 MΩ for the INST jack. You can expect other devices to be somewhere around these levels, though check their manuals for specific figures.
Extra gain is provided for the more sensitive MIC input in the form of a mic pre-amp (pre-amplifier). Manufacturers may make much fuss of the quality of their mic pre-amps as they can make a significant contribution to the level of background noise heard on the mic signal.
When using a dynamic mic via JACK [INST] the sound level is very minimal
For a dynamic mic you can use a mic transformer to boost the signal voltage. For example (not a recommendation for or against) this one will boost the signal voltage by 10x or 20 dB. A mic transformer doesn't require power.
For more gain you will need a separate mic pre-amp (another example). Some inline pre-amps use phantom power from the XLR input on the device they feed. Phantom power isn't available on unbalanced instrument jacks so you'll need an amp that (like this example) has its own power supply.
An electret or condensor mic will usually need phantom power which a pre-amp will need to supply. A mic transformer won't supply this. (Some electret mics have battery instead.) Dynamic mics don't need phantom power.
It does seem to be hard to find cheaper stand-alone mic pre-amps. A popular route for setting up a home studio is to use a mixer or USB audio interface with as many XLR mic channels as you need. This is a cheaper route than adding stand-alone mic pre-amps.