If you use an ATM card in Thailand, the bank who operates the specific ATM will have details of your name and account number. This transaction can be reported to the Thailand Treasury, who can then request under CRS details of ALL your card transactions carried out in Thailand. If there are any "suspicious" patterns you may be asked to explain source of funds. Failure to do this may see your card blocked for use in Thailand
DTAs exist for most western countries, I only know for a fact Australian pension is only taxable in Australia, and likewise the US is the same. So that's pretty clear. If you use your ATM card to withdraw cash, you are aware that your details are stored on the database of the bank whose ATM you are using? So the Thai bank has your name and card number. The bank can report this to the Thai Tax Office who can then request details under CRS of your history of withdrawals in Thailand. If you have a solid history of these, expect to receive a "please show source of funds". The electronic world we live in today, everything is traceable
The only thing I've noticed is the stupidity of people making comments in these threads. One dummy the other day said savings from ten years ago will be taxed. The stupidity of people unable to understand basic taxation concepts amazes me!
If you're bringing in money from investments you will need to declare it, and offset tax payable in Thailand with tax already paid elsewhere. Income of this nature is different to pension payments. Even if you bring in cash, you might have to declare it, same as ATM withdrawals and Credit Card purchases