Speaking generally you need a microphone preamplifier and then analog to digital converter, to get microphone input into a computer.
All-in-one-interfaces have both of these built in and, often, also have inputs to let you bypass mic preamplifier, for plugging in 'louder' devices...keyboards, digital music players, etc.
With these interfaces, and mixers, XLR generally means microphone input and 1/4" plug generally means line-level input (or instrument level).
Generally microphone inputs have a gain knob where line inputs do not. (just for a clue...)
In your current set up the Focusrite serves as mic preamp and converter.
In a mixer-based setup the mixer would effectively replace mic preamp, but continue to use the focusrite converter.
For that reason we want to plug into a line-level input on the focusrite.
The path is microphone > XLR cable > Mixer mic input ---- Mixer line level output > 1/4" cable > focusrite line input.
If you choose a mixer which has eq and compression, this will give you the same set up as you had before but with that additional functionality in the analog domain.
I.E. your voice can be made to sound louder (decreased dynamic range) and clearer (careful eq) before it gets to the computer.
PS. I'm making heavy generalisations for simplicity here.
Always read labels + manuals for certainty.