Alternatives to Google Wallet

You're not going to find something outside of official large companies. The reason they work at all is because the money never goes through your account during that transaction, but between the bank and Google/Apple/whoever. It's all handwaving and trust relationships. A typical tap goes through (basically) these steps.
Tap
The phone requests a unique one-time token from Google wallet, which is an intent to pay, signalling that the payment will be authorized
The POS system takes the token through the tap, sends it to their banking sofware
The banking system then asks Google/Apple/whoever "Hey is this token legit". They then validate it
The money transfers between Google -> Bank. (Google will probably front the actual deployment of money from your bank -> Google -> Their bank)
This is why it's the most secure, because nowhere in that process can anyone intercept anything about you, all they would get is a one-time use token, which is only good from that specific POS system. Even the POS doesn't really get much about you except for what Google/Apple responds with.
It's because of this behind-the-scenes transaction that makes it almost impossible to have an open source version. You would need to convince all of the existing banking infrastructure (who remember still use ~~csv files~~ "wires") to move money around in batches at night to not only implement Google, Apple, and Samsung, but also the open source app that is totally secure and legit, you can trust us.
Not only that, but you immediately open yourself up to issues with money too. Accepting money is easy, sending money out is just 3 1/2 headaches. Not only do you have the risk, you have security and compliance, you have agencies, you have auditing, FINRA, the SEC can get involved, and that's only in America. If you want to go international? Hoo boy, just take all of that neasea and personal liability and turn it up to 1000 because now you have to think about every country and their laws.
So, probably more than you wanted to know, but that's why you're not going to find one.
Source - worked FinTech for about 7 years, learned way too much about finance back then
- Cryptocurrency was supposed to solve this problem; instead we ended up with endless grifting and memecoins designed to leave ignorant rubes holding the bag.
  
  Lol "supposed to"
  Tell me another joke
  
  Eh, cryptocurrency was never supposed to solve any problem, it was a proof of concept that pariah states realized could enable them to avoid US sanctions. The "it's more secure than banks" talking points were conceived after the fact by the grifters.
- Exactly, modern banking is a permissionned network and its security is mainly based on access restriction.

Why invite any middleman into your transactions to spy on you at all? Tapping a credit card is the easiest thing to do.
- Strapping a card to the back of your phone is really the easiest alternative.
  
  Where I am, banks provide small NFC stickers for this purpose.

Wondering what Wero brings?

I know this won't resonate with people but gonna plant the seed anyway:
CASH MONEY, BABY
the OG privacy means of payment.

First - I use cash in the absolute majority of situations. Control and privacy are absolutely worth it. Second - don't banks usually have their own payment system anyway? At least mine doesn't have functionality tied to Google services (to acommodate Chinaphones that come without such, but accidentally acommodating Graphene, Lineage and others as well).

I had an objection to google pay for a while, and discovered Barclaycard (credit card in the uk) use an app which would act as a replacement for google pay. Worked perfectly. Of course then I found out how terrible barclays bank is for the environment, so I tap my co-op credit card now. Interestingly, google pay takes nothing when you use them to buy stuff, Applepay grab 0.25% as a transaction fee. Makes them billions per year, and the cost is shared by all of us.

You could set up some kind of NFC reader or dummy phone at home and then essentially wire its communication through the internet to your other phone and then have it Read a credit card that you leave on the reader at home. Although this is a little sketch.

Samsung Pay Apple Pay
If there are others I don't know about them. I use Google Wallet.
- I don't have an iPhone. Samsung Wallet only allows to add everything but payment cards
- Kinda weird there's not an open source alternative
  
  They'd need to collaborate with a multitude of financial institutions in whichever jurisdictions they'd want to operate in. That's just a level of resources most FOSS projects don't have. Plus, I can't imagine banks would be big proponents of open-source; where's the money in that?
  
  You can't just buy into API access for NFC payments. You got to pay somebody big bucks to let you on their network.
  
  Not really, it's not feasible for open source developers to get the kind of access required from payment providers to make it work.