By paying for this game on Itch you get the current B(beta) version and all future B versions before they go public. You pay ONCE, on Itch.
If you become a Patreon member on the Sad_Crab Patreon(monthly payment in support of dev)you also get all the B versions before they go public, but you ALSO get access to the A(alpha) versions that you cannot obtain on Itch.
The A versions are always subject to change, so that version type is more for those who want to have input/opinions on/about the game and support the dev further.
And lastly, you don't need to pay anything, and still get the Public version. The game is technically free, if you can wait. The majority of money paid to Sad_Crab is in support, and unless that changes in the future then the game is 100% free if you can wait until version 1.0.0.
EDIT 16-05-2024: Worth noting! If you pay an amount, at the point of this edit, that is BELOW $25 (so $24.99 or less) on Itch you get all current and future BETA versions. And if you pay (again at the moment of this edit) an amount that IS or is ABOVE $25, you get all current and future ALPHA and BETA versions.
And again to re-iterate, this is A ONE TIME PAYMENT ON ITCH.