You do not need anything at the border apart from your passport.
Some anti-Brexit scaremongers (I'm anti-Brexit too, but FFS these people were gloomsters) managed to dig out that according to Spanish immigration law, in theory they can ask you for proof of accommodation, spending money, etc. But these are rules that are only ever applied to people coming from a country where people need a visa for Schengen. So for example if you want to come from India and stay with your brother who is a resident of Spain, as part of the application process you need a letter from your brother (witnessed at the local police station) saying you will stay with him, you need a copy of your bank statements, etc etc etc. The consulate in your home country reviews all that before issuing the visa. The people at the border by and large don't even know that that requirement exists, and they certainly have no protocols for you to pull up your online banking and satisfy them that you have X amount of money in your cash ISA.
There have been about 100 million British tourist entries to Spain since Brexit and not a single one has been asked for proof of resources or accommodation. How do I know this? Because the tabloid media wheel out the "Watch out, you need all this stuff" story every week, and yet they have never found anybody that it actually happened too, even though the headlines would be an absolute gift ("NOW Sharia-loving jackbooted EuSSR bureaucrats PUNISH hard-working Brit families for Brexit with LUDICROUS spending money rules").
On the other hand, about once a week we see a story of someone who got drunk, fell off their balcony, is now in hospital, and is running a GoFundMe because they travelled without insurance. So by all means get that, and take a GHIC, but if you are asked at the border for insurance or your bank balance or proof of accommodation, I will pay for your holiday myself.
Now, you do need to fill in some basic stuff for Airbnbs. But it's nothing that you don't already have to provide for hotels in many European countries. And the only fines that can be issued if the data is incomplete are for the owners, not you. This despite recent lurid headlines in the UK media saying "NOW Sharia-loving jackbooted Spainish bureaucrats PUNISH hard-working Brit families for Brexit with LUDICROUS registration rules — you have to provide your bank account details and there's a 30,000 Euro fine for not complying!!!!!!!!!1!"). All total, utter bollocks, designed to scare you for clicks.