Something's definitely off here!
I would speculate that the septic tank pre-dated the widening of the concrete - but cannot work out why, other than sloppy work, they would have concreted over and then put manhole covers that serve no purpose (other than perhaps to fool people into thinking the job was done right!)
[If I understand correctly, you open either manhole cover, and just see the same concrete immediately beneath it?]
Certainly, pretty much regardless of the type of tank, you need to be able to inspect it. Soakaway types are not uncommon here, and are not as bad as they sound - and if they're working well, they shouldn't need emptying under normal use (= provided you're not putting loads more waste down the system than was envisaged when the tank was specified - and by the way, you'd need to minimise chemical-based products, bleach, etc, ideally using products labelled as septic tank suitable).
I suspect a plumber is unlikely to want to get into this (though some may). Typically (but location-dependent) your municipal water supplier may be able to send someone, or recommend someone; or there will be one or more private septic tank companies (emptying, installation, etc) serving your area. I'd speak to them.
This may just be a question of access for inspection - the tank may be perfectly functional and adequate, if only you could get into it!
If it needs substituting, consider its location. It's hard to be sure from the photos, but aside from being under a patio, it's relatively close to the house, and may be higher than and close to other features (pool, outdoor leisure space, cultivation). Replacement, if required, should take into account current regulations; and all such water systems should technically be registered with the APA (Portuguese environment agency) though few legacy systems, and many new ones, aren't.