Sidecar / pillion adaptable for standard push bike

Sidecars are going to be expensive because they are so unusual, and aren't produced in volumes that would permit economies of scale. They also dramatically change your bike's handling, of course.

The closest thing to what you're discussing that I can think of is the Xtracycle conversion kit. Still not cheap.

There might be a trailer that would work, but I am not aware of any bike trailers designed to accommodate full-grown adults.

I've ridden with one of the larger weight-limit child rear seats over the back wheel. The effect on stability is enough that I certainly wouldn't want an adult there.

A tandem puts the rear rider ("stoker") between the wheels so handling is a lot better. The stoker could pedal with no real effort, or you could remove the pedals or even the cranks (you may need to cut off the cranks because of the need for something to support the rear chainrings. I know of this being done in practice for a disabled teenager. Tandems are fairly common and come up second hand. The stoker generally has to be able to step over the frame.

There are tandem trikes which could be treated the same only they're more stable. Some are step-through for one or even both riders. They'd be expensive though, like a rickshaw, and too wide for a lot of bike infrastructure.

In many jurisdictions, trailers and seats for (non-pedalling) adults are outside the scope of legislation, which may make them effectively illegal.

In all cases even gentle hills will be much harder when one adult is propelling the weight of two. An electric motor may help, but it should be chosen with care for low-speed use, for example those used on electric cargo bikes.