First of all, Facebook and other social media sites can be a great way to find information, but at the same time this information is not always accurate, or it is not explained properly, or simple the same process is done in different ways in different areas of Spain. I know that sounds crazy, and if you are applying for the same thing the paperwork required should be the same everywhere in Spain. The reality is that I use 3 different national police stations to sort out paperwork for my clients and in each of them the officers ask for different documents to prove the same things.
So my first piece of advice is that you find someone that does the service within your area and ask them directly. If you prefer to deal with paperwork yourself, then go directly to your closest national police station and ask them what they want you to bring. This is the only way to avoid having to come back 3-4 times to do just one application.
For NIE and residencia, often people think that if they have a NIE they also have residencia, or they have residencia but think they only have a NIE.
NIE stands for Número de Identificación de Extranjero, which means foreign ID number. When you are in Spain and you need to deal with bureacracy, you need a number to identifiy yourself within the Spanish system. That is your NIE.
In order to be able to get one you need a real reason for it, for example: you have been offered a job, you are becoming self employed, you are buying a car, you are applying for a loan/mortgage ... Simply saying "I am moving to Spain" it is not enough of a reason, there are some police stations that will ask for confirmation in writing of your need for NIE, such as letter from the bank, car sale agreement, job offer, etc.
The NIE comes in a white A4 paper, and it doesn't expire, the number is forever. The actual piece of paper where the number is written (the NIE certificate) used to have an expiry date of 3 months from the date of issue, but this is not the case anymore. Your NIE does not expire.
Having NIE doesn't mean that you are resident in Spain, it means that you have a number that identifies you within the Spanish system. Nothing more. So if you have been living in Spain for 5 years, you have your NIE, and a job and you pay taxes, none of the above means that you are a resident here. It means that you live here, nothing more.
In order to be legally registered as resident in Spain you must apply for your residencia card. For non EU countries this is a must, and it has to be done within certain time period after your arrival to Spain.
However, for EU citizens, even though it is also a requirement to register as a resident, the system is a bit more lenient, and many people has never registered officially as residents in Spain.
Now Brexit has shown us the importance of being registered as resident. Our government has already agreed to grant residencia to all UK citizens that are residents here. But this means the ones that are registered as residents officially, not the ones living here but are not registered.
When you move to Spain you might need a NIE straight away, or perhaps you can apply directly for residencia if you comply with the requirements. If you have not received a NIE by the time you apply for residencia, you will be allocated one there and then. So if you are in a position to wait until you can apply for residencia, then you will be saving yourself time and money by applying only for residencia instead of a NIE and residencia.
This is quite a complex subject, and I could be speaking about it for hours, but I hope that I have managed to clarify some of your doubts.
