There is no such thing as 'illegal tier' — you're just trying to use Pokemon in formats where they are not allowed. Aurorus and Meloetta currently aren't available in Sword and Shield, so they appear as 'illegal' options for most Gen 8 formats. (If you don't choose a format, the teambuilder still uses the current gen dex by default.)
However, if you tell the teambuilder that you're playing a format like National Dex AG, then the 'illegal' warnings on Aurorus and Meloetta will go away because all Pokemon are allowed in that format. Similarly, if you choose Gen 7 OU, then they will both become available because they are allowed in that format. However, other Pokemon like Rillaboom and Inteleon will become 'illegal' because you can't use them in Gen 7 OU.
Note, nothing actually stops you from adding illegal Pokemon to your team. You just won't be able to queue the team in the ladder you nominated for it (or that the teambuilder defaulted to) because it won't pass the validation check.
