For the first part, I decided to break the problem into: 1. Reading the schematic, 2. Finding the numbers, 3. Finding the parts. This was useful for Part 2 as I could re-use my read_schematic and find_numbers functions.

Two things I typically do for grid problems:

1. Pad the grid so you can avoid annoying boundary checks.
2. I have a DIRECTIONS list I loop through so I can check easily check the neighbors.

Schematic  = list[str]
Number     = tuple[int, int, int]

DIRECTIONS = (
    (-1, -1),
    (-1,  0),
    (-1,  1),
    ( 0, -1),
    ( 0,  1),
    ( 1, -1),
    ( 1,  0),
    ( 1,  1),
)

def read_schematic(stream=sys.stdin) -> Schematic:
    schematic = [line.strip() for line in stream]
    columns   = len(schematic[0]) + 2
    return [
        '.'*columns,
        *['.' + line + '.' for line in schematic],
        '.'*columns,
    ]

def is_symbol(s: str) -> bool:
    return not (s.isdigit() or s == '.')

def find_numbers(schematic: Schematic) -> Iterator[Number]:
    rows    = len(schematic)
    columns = len(schematic[0])

    for r in range(1, rows):
        for number in re.finditer(r'[0-9]+', schematic[r]):
            yield (r, *number.span())

def find_parts(schematic: Schematic, numbers: Iterator[Number]) -> Iterator[int]:
    for r, c_head, c_tail in numbers:
        part = int(schematic[r][c_head:c_tail])
        for c in range(c_head, c_tail):
            neighbors = (schematic[r + dr][c + dc] for dr, dc in DIRECTIONS)
            if any(is_symbol(neighbor) for neighbor in neighbors):
                yield part
                break

def main(stream=sys.stdin) -> None:
    schematic = read_schematic(stream)
    numbers   = find_numbers(schematic)
    parts     = find_parts(schematic, numbers)
    print(sum(parts))

For the second part, I just found the stars, and then I found the gears by checking if the stars are next to two numbers (which I had found previously).

def find_stars(schematic: Schematic) -> Iterator[Star]:
    rows    = len(schematic)
    columns = len(schematic[0])

    for r in range(1, rows):
        for c in range(1, columns):
            token = schematic[r][c]
            if token == '*':
                yield (r, c)

def find_gears(schematic: Schematic, stars: Iterator[Star], numbers: list[Number]) -> Iterator[int]:
    for star_r, star_c in stars:
        gears = [                                                                                                                      
            int(schematic[number_r][number_c_head:number_c_tail])
            for number_r, number_c_head, number_c_tail in numbers
            if any(star_r + dr == number_r and number_c_head <= (star_c + dc) < number_c_tail for dr, dc in DIRECTIONS)
        ]
        if len(gears) == 2:
            yield gears[0] * gears[1]

def main(stream=sys.stdin) -> None:
    schematic = read_schematic(stream)
    numbers   = find_numbers(schematic)
    stars     = find_stars(schematic)
    gears     = find_gears(schematic, stars, list(numbers))
    print(sum(gears))