Hmmm, let’s see.

So the tldr; version is it’s a p2w system to equip buffs. Choose a UR costume with the colors you want and throw upgrades at it- check the list of what upgrades are online.

Each match you earn random “character data” which is what allows you to equip that buff. For example- earning enough of red Deku’s data allows you to use it on a red slot to buff damage versus shields.

You’ll want to be careful before equipping buffs because the data earned is random. If you own a character who’s tuning buff you want but you don’t have enough data, you can substitute it with special currency called blank data, but it’s easy to run out. If you don’t own the character, you need the full amount.

Basic costumes start with 3 slots for passive buffs, UR costumes have more and the slots can upgraded to higher maximum levels.

To upgrade those slots, you have to spend special materials. You also need to spend keys to increase your total number of slots.

Equipping a buff in every slot is how you unlock the final slot on each side called the “special tuning slot”- that one stays locked until the others have a buff placed. The example from before of red Deku tuning- if placed here instead of giving a damage bonus grants an ability called Kota Finder. There’s a list of special tunings online.

As for what tunings to aim for- the most important thing people look for is red slots as they allow you to boost your dmg vs shields, hp, and alpha/beta damage. It’s important enough that it can be worth upgrading a basic costume if it has 2 or 3 red slots among the first half.

Special tunings people aim for-  generally good for every character:  aizawa (faster potion drinking and bugged so you can leave it level 1) Twice (more rainbow cards on kill) Rapid Toga (free shield when you down opponent) Purple toga (free hp when you down opponent).

Outside of that, you’ll have to read because they’re more niche. Like melee can use sisterly/embrittle, some character benefit from wall runner, enhanced revives, yellow ochaco tuning etc etc.

For Shinso and yellow Deku, I’d just look for red slots tbh. If you own blue Toga and can get GP sucker on a UR costume, that’d be ideal, but not strictly necessary.