@Klara *****
no, Thai companies and schools both hire foreigners and supply the work permit and sponsor the foreigner for the visa. It does not involve having a significant amount of money nor does it require 'having a work permit already from somewhere else'-- the work permit is specific to the place that employs you, so there's not really such a thing as having a work permit for somewhere else.
What you're describing sounds like how some foreigners do attain a work permit, which is on a business visa and essentially opening your own company.
The more common route is to be hired, by a Thai company, for teaching or any other career that requires an English speaker or foreigner in a role, and then once hired they will provide everything the person needs in order to get a non-B visa (appropriate work visa) and they will also provide all the documentation needed for the work permit.
The only requirement for teaching is a university degree (in any subject / any major) and for other jobs, it depends.... But I know of foreigners employed by Agoda, or other similar online companies but if hired from the Thailand head office, they're employed legally, on a correct visa and given a work permit for that specific role at that specific company.