There are some standards for each part of the aircraft like actuators, sensors,... I could not find one for the flight control computer hardware. For example what are the environmental tests, or any standards for its connectors and ports,...?
In the US for commercial aircraft, SAE-ITC provides recommended design standards in ARINC Characteristic 701-1 Flight Control Computer System (FCCS).
The recommended environmental standards are in Attachment 6 of ARINC 701. This references environmental categories defined in RTCA DO-160. The issue is that ARINC 701 references an earlier version of DO-160 than the current DO-160G.
The main reason ARINC 701 has not been kept current (701-1 was the last update in 1983) is that the aircraft manufacturers consider the FCC an integral part of the aircraft and they provide the detail design specifications to their FCC supplier. The specifications typically reference DO-160 (whatever version is current when designing the aircraft). The actual categories and test levels are specified based on their analysis of the environment where the equipment is installed in the aircraft.
To summarize; the FCC hardware will have to meet the customer's specification which is usually in terms of RTCA DO-160 tests and categories. The actual levels are unique to each aircraft.
You need to look up MIL-PRF-38535J which specifies electronics performance requirements. Toward the late 90s/early 00s the "performance standard" or PRF, replaced MIL-SPEC for the manufacture of ICs and microprocessors, and production of components built to MIL-SPEC (which specifies physical construction requirements) was discontinued.
MIL-PRF doesn't require specific manufacturing specifications, only that components can meet a performance standard. This allows manufacturers to use commercial parts as long as they meet the standard, and they will find compliant components by performance testing individual parts from a batch in accordance with MIL-PRF, and set aside those that pass for use.
At the OEM I worked, our engineering organization was notified by suppliers in the early 00s that they were switching to commercial ICs/Mircroprocessors filtered by MIL-PRF testing for manufacture of various cards in black boxes because MIL-SPEC hardware was being discontinued, and if we didn't like it too-bad-so-sad there wasn't really any choice in the matter.
For individual bits like connectors and ports, a supplier will use whatever commercial parts suits their fancy, and the validation is done with when the certification testing (environmental, endurance etc.) of the black box is performed to satisfy the OEM's agreed-upon requirements.