If it’s ready made any general builder will be able to do the job. depending on the type of pergola you’re buying, if it’s bolted to the floor then yes you’ll need a foundation as such, but freestanding ones should be dug into the ground and held there with post mix!

I wouldn't use bolts and and brackets metpost etc...
I would dig out post holes with shovallers and once post holes are dug out ready set string lines to desired height and tape measure across to each post the same distance apart levelled and squared off with string lines.
Then add post crate into holes 2-3 bags in each will stop the posts frame from being blown down.
With bolts and brackets over time will move and pull out of there place, and rust or weaken from weathering.

It depends on the item which will have instructions on its suitability for fitting.
The pergola needs to be fixed to something, they are generally not free standing. Bolted to a wall and the supporting legs on suitable concrete bases.
If it is free standing putting the posts into post fix is a cheap way but the posts will rot in time and the pergola will collapse.
Metal post sockets with rods that go into post fix are more suitable, not the general met posts but appropriate decorative supports.

A timber pergola should ideally have 125 x 125 or 150x150 posts concreted into the ground. Being a see through structure the posts don't need to be in more than 400mm as wind wont affect it! Its more just to keep the posts in place as you fix to them. Some are free standing and others are 2 posts and attached to a house wall. If the structure is the more flimsy type (not like ours) and you want to build it above ground first, then just mark your holes when built then drop the legs (posts) in and bubble up and fill with a compacted semi dry mix of 4-1 (ballast)