Magento 2 - We have multiple options for separating your custom attributes like
USING COMMA
attribute_code1 = custom_attribute_value1 , attribute_code2 = custom_attribute_value2 , gon on...
USING VERTICAL BAR
attribute_code1 = custom_attribute_value1 | attribute_code2 = custom_attribute_value2 | gon on...
USING COLON
attribute_code1 = custom_attribute_value1 : attribute_code2 = custom_attribute_value2 : gon on...
We have Multiple value separator option in admin panel, below-attached screenshot for your reference

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Magento 2 - When you are export product CSV, created product custom attributes comes under "additional_attributes" column in CSV file.
If you have multiple custom attribute, which would be separated by commas in same "additional_attributes" column
additional_attributes column in product csv file
Product 1 : attribute_code1 = custom_attribute_value1 , attribute_code2 = custom_attribute_value2 , gon on...
Product 2 : attribute_code1 = custom_attribute_value1 , attribute_code2 = custom_attribute_value2 , go on...
Finally, you can proceed with normal magento2 import process, you can see respective product attributes value is updated...
Notes : Magento 2 custom attributes value comes under "additional_attributes" column in csv file