Avantis has fx sends automatically created for each FX slot that is set as send/return (as opposed to insert).
The advantage of this is that A. they are automatically created and connected up properly and B. they have a short cut button on the fx send to the FX settings.
Disadvantages: they eat up a mix just like an aux, but they have less processing options than an aux plus you are unable to send to multiple FX from the same FX send, which is something I do to save on buses/mixes.
When you use an aux as an FX send, you have to go to the FX page, click on the ‘back panel’ of the FX you want to use, and select that aux as the input to that FX.
I usually use the dedicated FX return channels as the returns, but I’m sure you can also rout them to channels in stead, which gives you more processing options.
If you’re not at the board, just download director software to practice setting it up.
It’s not rocket science, but it helps to have done it before you have to do it on the spot.