There is no single answer to this question. As a locksmith, I will share the following:
I refer auto repair to auto repair specialists. In turn; they call me in for auto lock and programming.
That said and done, there is crossover between the two. I often am called for diagnostic work to mate ECUs, reflash ECUS etc.
There is no one magic bullet device. Some devices can program keys, but you need a seed code or pin code, that the tool can not produce. In other instances, Maxidas can program the keys.
As far as BMW, Mercedes, Fiat and nearly ALL new applications, you will encounter the need to be a NASTF Automotive Security Professional for access to the "security side" of each manufacturers' site. They generally allow this only for locksmiths. EVERYONE can access the auto repair side these sites. Nearly everything new is being phased over to a J-XXX pass-through. Kudos to the firms ( Such as Autel ) that reverse-engineer, and make vehicles serviceable off-line.
So, to become an Auto Locksmith, and have the capability to do all cars, you will need:
1.ALOA Membership or equivalent
2. 4 programmers est $25,000.00 plus annual updates at about $2500.00/year
3. At least 2 Key machines est $18,000.00
4. Nastf membership, which is subject to a very thorough background check and verification of business entity
5. Subscriptions to each manufacturer ( they are annual, monthly or short term, and NOT included in the mechanical repair subscription. )
6. You can not give employees access to logins to mfr sites, each employee needs a NASTF membership to have access
7. Inventory of keys, remotes, prox etc $25,000.00
To those that desire to become locksmiths, welcome aboard. But do a cost-benefit analysis first. I am sure the interwebs will tell how to do it all cheaper, but we are all working to being BETTER at what we do by going deeper, not wider and shallow.