That's correct, first you must apply for a Non-Immigrant Visa. There are two options for retirement for people aged 50 years and over.
There's rhe OA visa which gives an initial one year permission to stay, and the O visa which gives an initial 90 day permission to stay.
The "Non-OA" can only be applied for in your home country and has more onerous requirements than the "Non-O". The Non-O can applied for either in your home country, or at your local Immigration office after you arrive in Thailand.
If you choose the "arrive first" option then you can enter Thailand either visa exempt or on a tourist visa. After you arrive you can go to your local Immigration office and apply to convert your visa exempt or tourist visa to a Non-O.
The conversion process requires you to show B800,000 in your Thai bank on the day you apply AND prove that the funds came from overseas. Except for Pattaya immigration office who want to see that the funds have been there for at least two months.
The Non-O visa gives you a 90 day permission to stay from its date of issue and is marked USED as soon as it is issued. Towards the end of the 90 days you can apply for a one year extension of stay.
To obtain the extra 1 year stay you still need the B800k in the bank but you don't need to show that it came from overseas. Instead you must prove that it has been in your Thai bank account for at least 2 months.
Once your extra 1 year permission to stay has been granted you must keep the B800k in your bank account for 3 months afterwards. Then you can withdraw funds if required but you cannot dip below B400k.
After that, if you want to stay another year you must top-up your back account to B800k at least two months prior to the expiry of your permission to stay.
I think I covered most of the salient points but I'm not a master like Ubon Joe so no issues if anyone wants to flame me for any factual or grammatical errors ????