My 50 yr old siding is in good condition, but there are places where the end of a piece has bulged or popped slightly. Also, the base of very bottom row tends to be loose from the house. This allows critters inside. How do I secure them? Tank you.

You can buy aluminum trim nails. They come in white, brown or beige color.
You can tack the corners if they are loose.
As for the bottom course of siding, it sounds like the starter strip is loose. The only way is to use those trim nails but nail low or high on the siding panel so the nails won't be too noticeable.

Of course if your siding was vinyl, the fixes would be much more easier.