Hairline cracks can just be from slight movements in the brickwork over time. Generally through expansion. It's quite simple to fix, just a matter of cutting the joints back a bit and then refilling with mortar and pointing again.

I think anything over 7/8mm there could be a structural issue.

check for subsidence and lintels over windows and doors if there is no obvious reason the it could be expansion and contraction over time requires joints grinding out and re pointing

If you’re worried you could fix tell tells over some of the cracks. This would tell you over a period of time if there is still movement.
If no move movement, happy days, just a repoint job, if the cracks get bigger it’ll be a structural job, not so good.
Hope it’s just a repoint job, good luck

You need an onsite inspection. Its too vague. .ythe otjer answer was ok but it may be necesssry to femove the bricks and replace. It could also be a lintol problem.

There is no need to be worried, brickwork and the bed joints and crack over time, due to expansion and movement, it can be repaired by removing the old mortar and re pointing with new. If cracks become bigger than 6/7mm then it could be structural and need looking into further