They usually put a form in with the paperwork you get about filling out your bank details for payments and all that.
Being in a union won't change anything for you individually. You pay/breaks etc isn't affected by you swapping unions. You'll get the same pay/breaks etc as everyone else who works for the same business regardless of what unions y'all are in.
Unions just use the money you pay them to fight for better conditions for everyone - so at woolies you have an enterprise agreement (you can probably find it online), that's what determines how much you get paid, what breaks you get, all that sort of stuff. Enterprise agreements are decided between the employer (woolies) and the unions (sda and raffwu).
The more people in the union, the more leverage they have to get the employer to agree to things they want. Like if the SDA had 10,000 members and raffwu has 100 and raffwu wants half hour breaks but the sda is fine with 15 min breaks, raffwu is probably gonna get ignored because the employer can appease the majority with just giving you 15 min breaks. The more numbers the union has the easier it is for them to fight for better conditions - but those conditions are for everyone, it's not variable depending on whether you individually sign up.
Unions are most valuable when those enterprise agreements are being made - you want the union that's meant to be fighting for what you want to actually be doing that and not be corrupt.
On an individual level unions can help with disputes with employers or helping with complaints but that's probably not something you need to worry about right now.